Lorimar Inc., producer of the popular "Dallas" television series, said yesterday that it would be acquiring "more than $15 million but less than 15 percent" of Warner Communications Inc. stock "for investment purposes only."

The publicly held Culver City, Calif., media company, best known for its television shows such as "Falcon Crest" and "Knots Landing," has aggressively sought to expand its media holdings, acquiring advertising agency Kenyon & Eckhart in 1983 and recently buying the Bozell and Jacobs advertising agency for about $40 million. It unsuccessfully sought to acquire Multimedia Inc., the South Carolina-based communications company, earlier this year.

However, Lorimar spokeswoman Carol Henry said the company was not seeking to acquire Warner and filed a Hart-Scott-Rodino antitrust statement with the Federal Trade Commission, required in certain situations when companies are positioned for a merger, because "we were advised to by our attorneys."

Warner Communications, which was a takeover target of Australian media owner Rupert Murdoch two years ago, said the company had been "assured" that Lorimar would not seek a stake in the company beyond 15 percent, according to Warner spokesman Geoff Holmes.

Warner, which had a 1984 net loss of $586 million on revenue of $2.02 billion, has been restructuring itself since it sold off its money-losing Atari video games and computer subsidiary two years ago.

The New York-based movie, television production and publishing company announced that it would buy out American Express Co.'s half share of their Warner-Amex Cable Communications Inc. joint venture for $450 million.

Earlier this week, Warner said it would sell its share of the Showtime/Movie Channel cable service and the MTV Networks Inc. music video and children's programming channel to Viacom International Inc. for $500 million plus warrants for Viacom stock. Warner said the sale would be used to finance the cable systems acquisition.

"Every time Warner Chairman Steven Ross does an exit strategy, he does brilliantly," said Mario Gabelli of Gabelli and Associates, which represents investors in Warner and in Chris Craft Industries, the largest Warner shareholder. Warner stock has risen to more than $30 a share from a January low under $20 a share. "You have a company selling at $30 a share with assets that could be worth as much as $45 a share," Gabelli said.

Warner stock rose 1 1/2 to close at 31 1/4 yesterday, while Lorimar dropped 1/4 to 34 7/8.