The traveling snake-oil salesman of days gone by may have been replaced by some of the modern-day purveyors who come into town, promote their wares at visiting shows in large exhibition halls and move on.

One such show, the Health Horizons Positive Living Expo, was held in Washington for two days in February with about 150 exhibitors selling purported remedies for everything from cancer to acne.

But times have changed since the days of the itinerant peddler, whose sales were governed by little more than the rule of caveat emptor. The Washington exhibitors unwittingly made their pitch to undercover officers of the Food and Drug Administration's new Fraud Branch, who took a dim view of some of the goods.

Last month, 13 of the exhibitors were sent warning letters citing them for violating the law by selling drugs that had not been proven safe and effective, for misbranding, for false and misleading advertising and for giving inadequate instructions for use.

FDA says the "drugs" included bottled water for arthritis, urinary and heart disorders; a hydrogen peroxide bleach for cancer, kidney stones, multiple sclerosis and inflammatory diseases; an extract of a cow mammary fluid for stimulating the immune system; and various algal, herbal and mineral concoctions for a host of other medical purposes.

"We followed those that were clearly interstate and those with the most egregious claims, products where clearly if the individual relied on them to the exclusion of proven therapy, harm could be done," said FDA spokesman Bruce Brown. FDA has jurisdiction only over products in interstate trade.

"We have received a large number of responses from the companies or their attorneys," said Bill Nychis, acting head of the FDA fraud squad. "Some companies have promised not to pursue marketing of the products any further," he said, while others promised to talk with FDA officials.

Investigation of fraud and abuse in selling worthless or dangerous products is at the heart of FDA's mandate and was part of the impetus for getting the government involved in regulating foods and drugs nearly eight decades ago.

But until last September, says Brown, "the agency had no employes, field or headquarters, who worked full time bringing fraud cases forward."

Now seven employes work out of FDA headquarters, under the Office of Compliance in the National Center for Drugs and Biologics, looking for targets and coordinating with the agency's 150 field offices.

They have taken action against look-alike drugs, herbal products for cancer therapy, and diet pills and products for melting off weight or getting rid of cellulite. They did the ground work for the current set of cases by visiting a health expo in Pittsburgh last fall, "gathering information and reflecting on what to do about it," Brown said.

Tracking down health fraud is often complicated by the fact that what is written on a product may differ from what the sellers or distributors say about it.

By posing as customers, they heard the sales pitches and openly taped them (sometimes over the objections of the promoters, who were selling their own cassettes, Brown said). The investigators also collected promotional literature and bought some products.

At least one of the products was well-known to the FDA and demonstrates the difficulty in controlling such sales.

Two exhibitors were selling Bio-Line Catalyst Water, promoting it as a treatment for arthritis, heart disease and urinary disorders. According to Brown, the product contains sulfated castor oil, rock salt, lignite, sodium, metasilicate, calcium chloride and magnesium sulfate but is 98 percent water.

The bottled liquid comes from a nationally franchised firm in Minneapolis, but is basically a reincarnation of Willard's Water, made by a Rapid City, S.D., firm that was shut down by the FDA in 1982.

Brown says the original formula for "carboneaceous activated water" came from Dr. John Wesley Willard, a retired professor of chemistry at the South Dakota School of Mines and Technology. The FDA stopped production of Willard's Water through a court order, but "other folks simply carried on," Brown said. "Legally, it's a separate company and a separate product. We would have to start from scratch."

The Bio-Line Catalyst Water exhibitors were among the 13 sent warnings.

The fraud squad plans to visit further health expos, but they aren't saying which ones. If they find further violations among those warned, they say they will have further grounds for taking stronger regulatory action, from seizures to injunctions.