An arrest in Greenbelt yesterday gave new meaning to the axiom, "Only your hairdresser knows for sure."

In a sting operation, the FBI arrested Roni Jacob Shinyder, who allegedly distributed about $300,000 worth of counterfeit Nexxus shampoo and conditioner to drugstores across the country. Real Nexxus shampoo can be obtained only from hairdressers, because the company sells solely to beauty salons and barbershops.

An additional $1.2 million of fake Nexxus products counterfeited by other sources has been confiscated by the company during the past 14 months. The goods probably are being manufactured in the Los Angeles area, company executives said.

Fake Nexxus shampoo, which has been found in the Washington area, contains a high amount of bacteria in some cases and could cause eye infections or sores on the scalp, according to Nexxus Products Co., a private company founded in 1979 that sells skin-care and shampoo products.

"In one laboratory test, the amount of bacteria in the shampoo was too numerous to count," said Rose Marie Demourkas, vice president of sales for the Santa Barbara, Calif., company. "Customers who wrote us complaints said they used the shampoo for years and suddenly it is making their hair horrible and causing itchy scalps."

Genuine Nexxus products are packaged with a raised Nexxus logo on the shoulder of the bottle and a blue-and-white cap. Some of the counterfeit products have a brown cap and don't have a raised Nexxus logo.

"No counterfeit products have been found in hair-styling salons," Demourkas said.

Although many consumers think that Nexxus bought at a pharmacy or other store instead of at a hairdresser is cheaper, the counterfeit product is more expensive in some cases.

"We spend a tremendous amount on advertising, but we sell only in salons," Demourkas said. "Demand for the product in drugstores is high because consumers don't know where to buy it. So when someone comes into the store and says they have Nexxus to sell, many of the merchants seize on the opportunity."

Nexxus has been raiding stores and searching for a suspect for more than a year since its products began showing up outside of hair salons, and the company received a tip from a retailer it sued linking Shinyder to the bogus goods.

Shinyder is alleged to be responsible for the distribution of counterfeit Nexxus products in at least six states. Those goods have wound up in drugstores, flea markets or cosmetic stores in virtually every state, according to James L. Bikoff, counsel to Nexxus and president of the International Anticounterfeiting Coalition.

The FBI arrested Shinyder and another man, whose name was not disclosed, in a Hertz rental truck in Greenbelt after getting a tip from a distributor's friend who was in the beauty supply business. The FBI seized at least $40,000 -- or 500 cases -- worth of counterfeit Nexxus products.

Investigative and legal costs to Nexxus exceed $1 million, said Stephen J. Redding, the company's executive vice president. "The costs swallow you up," Redding said.

About 100 civil actions have been filed against sellers and distributors of the counterfeit shampoo during the past year. If prosecuted and convicted under the Trademark Counterfeiting Act of 1984, an individual faces a maximum of five years in prison and a fine of $250,000.