NEW YORK, JAN. 13 -- The Securities and Exchange Commission is investigating possible insider trading in Sterling Drug Inc. stock options before the announcement last week of a $4.2 billion takeover bid for Sterling by F. Hoffmann-La Roche & Co. of Switzerland.

The probe followed a referral from the American Stock Exchange, which noticed unusual trading in Sterling stock options prior to the takeover announcement, an Amex spokesman said.

The targets of the investigation appear to be one or more investors who routed trades in Sterling options through Switzerland. It was unclear whether the investors are Swiss residents. The trades under investigation were executed in the United States by the brokerages PaineWebber Inc. and Drexel Burnham Lambert Inc.

"PaineWebber is cooperating fully with the SEC and we have no further comment," a PaineWebber spokeswoman said.

Drexel spokesman Steve Anredder said the Sterling trades being investigated by the SEC were "unsolicited trades which we executed as a broker."

Routine surveillance of options trading volume prompted the preliminary Amex investigation. "In our inquiry, we found unusual volume in the options trading," said Jack Krug, an Amex spokesman. "We immediately notified the SEC and they started the whole deal."

A spokesman for the Swiss Justice Ministry revealed today that the Swiss government had received a request for cooperation in the probe from the SEC on Monday morning. It appeared the SEC invoked a 1982 agreement under which Swiss banks pledged to cooperate with U.S. investigations of suspected insider trading.

Michael Mann, the SEC lawyer who supervises the commission's dealings with foreign governments, declined to comment, as did a Sterling spokesman. A spokesman for Hoffman-La Roche told the Associated Press in Switzerland that it would cooperate with the U.S. investigation.

Stock options permit investors to speculate on the direction of a company's stock price while putting up a relatively small amount of money. Heavy trading in Sterling call options prior to the Hoffman-La Roche takeover announcement prompted the Amex's initial inquiry.

An investor purchases call options when he thinks the price of a company's stock will go up. If an investor had bought Sterling call options immediately prior to Hoffman-La Roche's $72-per-share takeover bid, he could have profited enormously from the sudden rise in Sterling's share price.

Sterling has so far resisted Hoffman-La Roche's takeover bid and earlier this week filed a lawsuit accusing Hoffman-La Roche of various improprieties, including insider trading. Such lawsuits are typically filed during takeover battles.