In its next $10 million worth of advertisements, the makers of Listerine mouthwash must tell consumers that the product "will not help prevent colds or sore throats or lessen their severity."

The ruling by a federal appeals court here yesterday upheld the right of the Federal Trade Commission to order such corrective advertising for a product whose ads in the past have made false claims. For decades, the makers of the 100-year-old mouthwash product had advertised that it was beneficial for colds, cold symptoms and sore throats.

Government attorneys said the 2-to-1 ruling by the U.S. Court of Appeals for D.C., which traced the history of past government orders concerning misleading advertisements ranging from clocks to baking powder, is a landmark decision in the field of consumer protection.

In the opinion Judge J. Skelly Wright said, "To allow consumers to continue to buy the product on the strength of the impression built up by prior advertising - an impression which is now known to be false - would be unfair and deceptive."

The court majority did require the FTC to drop from its order to Listerine the phrase that noted that the new ad was "contrary to prior advertising."

Wright said that such a phrase would only serve either to attract attention that the advertisement contains a correction, or to humiliate the advertiser, Wright said Listerine should not be forced to use what he called a "confessional preamble" but merely to make the corrective advertising speak for itself.

The Listerine legal battle began five years ago when the FTC filed a complaint against the company because of its claims concerning its defectiveness against the common cold.

After four months of hearings and 40000 pages of documentary evidence including exhibits and testimony of 46 witnesses, the agency ordered Warner Lambert Co., the makers of Listerine. to stop saying that its use would treat or prevent common colds.

At the same time, the company was ordered to use corrective language in its next $10 million worth of advertising - which approximates the annual advertising budget for Listerine.

Warner Lambert appealed the FTC order. The manufacturer said the agency's findings were not supported by the evidence, that corrective advertising was beyond the FTC's authority and violated the First Amendment.

The appeals court dismissed the firm's contention the FTC ruling was not based on sufficient evidence that the product did not help treat the common cold.

It said that although the product might be an effective mouthwash and breath freshener, there was no evidence to support the fact it is an effective cold remedy.

Listerine may kill millions of germs on contact in the mouth, the court noted, but colds are caused by viruses and not by surface germs that would be reached by the mouthwash.

The court also rejectd the argument by Warner Lambert and various advertising groups that corrective advertisements violate the First Amendment.

Further, the court said there was clear precedent for the FTC to order the corrective advertising.

It pointed out that the government had taken similar step in the past in forcing persons who treat baldness to tell prospective customers that they could not help persons with hereditary baldness.

In another case, Geritol was forced to disclose that it was only helpful to persons suffering from iron deficiency anemia. A baking powder company was forced to disclose that it no longer contained cream or tartar after 60 years of bragging that it was better than other baking powders because it did, the court recalled.

And, the company that purchased the name of the Waltham Watch Company was forced to tell its customers that the watches and clocks it sold were not made by the famous old Waltham company Massachusetts but were in imported, the opinion continued.

In citing those cases, the court wrote that Listerine had built up over the years a widespread reputation concerning its use in treating colds. "When it (is) ascertained that that reputation no longer applie(s) to the product, it (is) necessary to take action to correct it," Wright wrote.

Chief Appeals court Judge David L. Bazelon agreed with Judge Wright.

Judge Roger Robb dissented, saying the FTC did not have the authority to order corrective advertising and that, even if it did, such an order was not necessary in the Listerine case.

Robb said Warner - Lambert has already stopped promoting Listerine for the use of colds and that any corrective advertising will relate only to past ads instead of current ads in which the product's breath-cleansing and mouthwash properties have been emphasized.

"Thus the commission and the majority would forbid the publication of truthful advertisements of Listerine's effectiveness unless coupled with a disclaimer relating to uses advertised in the past." Robb said.