Zack Jacobs and his fiancée thought they'd found their dream house -- an old mansion in Miami's Morningside area, but they didn't like the look of a mildew patch on a ceiling, the roof that leaked during heavy rain, the moldy edges around the kitchen cabinets.

So they called in the dogs -- Mold Dogs, to be exact.

Tootsie and Snickers, who belong to the Mold Detection Service of Miami, quickly sniffed out the fungus that had riddled floorboards, walls and ceilings. Jacobs backed out of the contract in a hurry.

''We were very much into get ting this house,'' said the Miami Shores kitchen wholesaler. ``But it was unhealthy. There were two rooms that were off the charts. They were unliveable.''

This pair of beagles and their mold-sensitive snouts are in increasing demand, say their owners David and Jennifer Leshner, siblings who started Mold Detection Service in January out of, well, puppy love.

''Being dog lovers, we'd been looking for a business with dogs -- a pet store, dog walkers, anything,'' related 32 year-old David Leshner, a former investment portfolio manager.

But it was an Animal Planet show about the 20-year-old European tradition of using dogs to root out mold that caught Leshner's attention.

After researching the idea, he and his sister, a 29-year-old lawyer, threw their careers to the dogs and ordered two pooches trained in mold detection from the Florida Canine Academy in Tampa for $15,000 each. They also got themselves certified as mold inspectors.

With the American Society of Home Inspectors estimating that 38 percent of U.S. houses harbor mold, mold detection is turning into a big business across the country as homeowners are becoming increasingly aware of the fungi and its ill effects.

Mold, which is caused by damp conditions and can grow in as little as 24 hours, is commonly linked to allergies and respiratory ailments.

The strain known as toxic or black mold has been blamed for more serious illnesses ranging from bleeding lungs to reduced cognitive skills and even death. Children, the elderly and people with low immunity are especially vulnerable.

But many in the construction and insurance industries, which are being hit with thousands of mold-related lawsuits and claims, say there is little scientific evidence to support the allegations of noxious health effects. The U.S. Centers for Disease Control is currently conducting a study designed to end the debate.

What is more commonly accepted is mold's destruction of buildings. The microbes particularly feast on drywall and more absorbent building materials, which makes new homes susceptible as well as old.

With its high humidity and booming real estate development, South Florida is a prime climate for mold, and mold hunting. ''It's a perfect place to open this business,'' David Leshner said.

And Tootsie and Snickers, 2-year-old females who were res cued from the Tampa Humane Society by the Canine Academy, are ideal employees. They're happy toiling for doggie treats and don't complain about working overtime. The Canine Academy chose the pooches for their friendliness toward people and their keen olfactory sense.

''To them, it's a game,'' David Leshner said. ``When they know it's time to work, they get really excited.''

When David Leshner gives the command ''seek,'' the dogs start sniffing around the room. They're trained to reveal 18 types of mold. At ''show me,'' they point with their noses, often in a nodding motion, to the location where they have detected the fungi. If it's in the floor, they sit on the spot.

After the dogs have signaled the location of the mold, the inspector takes an air sample by inserting a tube inside an electrical outlet, air vent or other opening. The tube is attached to a pump to pull the air into a trap that is later sent to a lab for testing.

So far, David Leshner said, the dogs have never failed. ''You can't hide mold from them,'' he noted.

Other detection methods involve cutting out pieces of wall for testing, which can be costly, slow, not very accurate -- and can even spread mold by exposing spores to the air, David Leshner said.

Mold Detection Service charges by the square foot; a 1,000-square-foot apartment would typically cost $250, plus lab costs.

Gwen Dermis, a financial consultant who is five months pregnant, considered it well worth the expense. She called Leshner to inspect her Normandy Isle home, which had been flooded in the past, after seeing a television segment on mold.

The dogs confirmed her fear of a colony of mold in the bathroom and the room destined for the new baby.

''I thought it was better to find out now than after the baby's born,'' she said. `We're now taking out the walls, raising the floor, cleaning with anti-bacterial scrub, putting in anti-mold plasterboard. The dogs were wonderful.''

As Dermis and many other Florida homeowners have found, the problem is that insurers are not required to cover repairs from mold damage, which can be expensive and can include replacing walls, floors and ceilings.

Wanting to sell separate policies for mold, one company -- State Farm -- is battling with the state Office of Insurance Regulation to exclude mold liability from homeowner's coverage altogether.

An administrative law judge last month agreed that State Farm is not obligated to cover mold damage, but state regulators will likely appeal that finding, said spokesman Bob Lotane.

The state, however, has approved other companies' requests to limit mold payouts to $10,000, Lotane said.

The fungal infestations are fast turning into an issue in real estate transactions, also.

A bill is pending in Congress that would require a mold inspection for any home that is the subject of a federal loan.

And more buyers are becoming like Zack Jacobs, who vowed that he won't close on a home unless it has passed a mold inspection, even if it's brand new.

''Who do you sue? The builder, the seller, the city?'' he asked. ``I'm going to do this every time I bid on a house.''

It all adds up to a bright business outlook for Mold Detection Service. The company has already ordered a third dog from the Canine Academy.

She'll be another furry addition for the menagerie; after 5 p.m., the dogs go home with Leshner and turn into playful pets.

''I thought this was crazy; crazy enough to work,'' Leshner said. ``Our friends laughed, but now they're telling everyone how amazing this is.''