Only three days after CBS Inc. appeared to have defended itself successfully against a takeover attempt by a conservative group, Wall Street arbitrageur Ivan F. Boesky put the network back in the spotlight yesterday when he disclosed that he owns 8.7 percent of the company.

The disclosure by Boesky came after the close of trading on the New York Stock Exchange. He said in a Securities and Exchange Commission filing that he paid about $247 million for his 2.6 million shares, purchased at prices ranging from $78.50 to $111 a share between Feb. 8 and yesterday.

Boesky paid an average of $95 a share for his CBS stock, which closed yesterday at $108, down $1.75. Boeskey's disclosure confirmed rumors in recent weeks that he has been acquiring CBS stock.

Boesky, Wall Street's biggest independent speculator in takeover situations, said he does not intend to seek control of the company. However, he put pressure on CBS management by saying that he will encourage the company to take actions that would result in a higher stock market price, which means he would support a takeover bid for CBS at a premium price.

CBS said it could not comment because it had not seen Boesky's SEC filing.

CBS has been fighting for its life since Jan. 10 when Fairness in Media, a conservative group supported by Sen. Jesse Helms (R-N.C.), said it would encourage conservatives to purchase CBS stock. FIM said its strategy was to organize a large group of individual stockholders to influence network news coverage at CBS, which the group said has a "liberal bias."

Last week, FIM said it would not launch a proxy fight at the CBS annual meeting on April 17. However, while CBS claimed victory in its battle with FIM, FIM officials said they would carry on the fight to influence the network.

Wall Street analysts believe that what began in January as an attempt by Helms to eliminate a "liberal bias" in CBS news coverage has made the communications giant more vulnerable to a takeover by a third party. Analysts said the FIM bid has drawn attention to the company's stock market price, which is well below the value of its assets.

Speculation in CBS stock has increased since the announcement on March 18 by Capital Cities Communications Inc. that it has agreed to acquire American Broadcasting Companies Inc. for more than $3.5 billion.