Unocal Corp. today stepped up its campaign to prevent Mesa Petroleum Co. Chairman T. Boone Pickens Jr. from gaining control of the company by filing an antitrust suit against Pickens and his investor group.

Unocal is the target of a $3.4 billion bid for control by Pickens, who already owns 13.6 percent of Unocal's stock. The company has said it will oppose Pickens vigorously.

The company has sued Pickens for alleged violations of securities laws and has changed its own corporate charter to make a takeover or change of control more difficult.

Unocal's antitrust suit filed today in the U.S. District Court for the Western District of Louisiana alleges that Mesa has violated the Sherman Act by taking steps to limit its competitors' activities in exploration for oil and gas. The suit says Mesa's takeover attempts and restructuring efforts in the oil industry have diverted funds away from oil and gas exploration to the benefit of Mesa and other parties, including two of Pickens' partners in the Unocal bid -- independent oilmen Cyril Wagner and Jack Brown.

The suit alleges that Pickens' takeover battles in recent years with General American Oil Co., Cities Service Co., Superior Oil Co., Gulf Oil Co. and Phillips Petroleum Co. all have led to greater corporate debt, which has limited exploration expenditures for the surviving operations of those target companies.

Pickens, who has profited handsomely from his unsuccessful takeover attempts by selling stock he acquired for a profit, has said repeatedly that he believes the domestic oil industry needs to be restructured because most of the major oil companies have wasted money unsuccessfully looking for new oil reserves.

The Unocal suit suggests that Pickens' efforts have restructured the industry, leading to cutbacks in exploration by oil companies that have benefited Pickens and his partners, who now face less competition when they explore for oil and gas.