One of the insurance companies trying to buy Equitable General Corp. of McLean dropped out of the bidding yesterday, and another that had dropped out earlier sued Equitable General for $15 million.

The two remaining firms seeking to merge with Equitable General are revising the terms of their offers "in a manner favorable to Equitable General shareholders," the company announced in a day of new moves in the lengthy merger battle.

Dropping out of the bidding was Liberty National Life Insurance of Birmingham, Ala., which had bid $50 a share for the local insurance company.

Liberty National said it had waited six weeks for a reply to its offer and had been forced to restrict its investment operations while waiting. It said it was withdrawing the offers so it could "return to normal investment operations."

Still pending are buy-out offers from Great Southern Corp. of Houston and Gulf United Corp. of Houston, both insurance holding companies.

It was unclear yesterday how their merger bids would be affected by the $15 million lawsuit filed against Equitable General by American General Insurance, a third Houston company.

American General acquired about 10 percent of Equitable General's stock a year ago and applied to the Virginia Corporation Commission for permission to expand its holdings to 21 percent of the company.

When Equitable General resisted the takeover, American General agreed last January to sell its 315,000 shares of stock back to Equitable for $32.50 a share.

In the lawsuit filed in U.S. District Court in Richmond, American General claims it lost potentilal profits of $15 million on the stock sale because of misrepresentations by Equitable General.

Accusing Equitable executives of violating federal securities law, the petition claims Equitable falsely stated it was not involved in merger talks with any other company and "concealed the true nature and status" of those talks.

American General claims it lost $5 million by selling for $32.50 a share stock that could be worth $50 if the merger is completed.

American General demands another $10 million in damages it allegedly suffered by not carrying through on its original plan to buy 21 percent of Equitable General when the stock was selling for about $23.50 a share.

The unsuccessful merger partner claims it should either be given a chance to buy the shares back, or be awarded cash damages for the profits it lost.

Describing the legal claims as "totally without merit," Equitable General said it has referred the lawsuit to its attorneys.

Equitable General had already been sued once over the purchase of the shares from American General. That lawsuit was filed by a group of dissident shareholders who claimed the $32.50-per-share price was too high and demanded the company rescind the sale. The dissidents have been silent since subsequent merger bids pushed the price of Equitable General shares to nearly $50. In over-the-counter trading yesterday, the stock closed at 44 1/4 bid, 45 asked.

Equitable General said yesterday its financial consultant, First Boston Corp., is still studying the offers from Great Southern and Gulf United.

The company said the bidders were contemplating changes in the terms of their offers, which involve purchase of some shares for cash and others for notes or preferred stock. The deferred payment provisions have been demanded by Equitable General insiders who face substantial capital gains taxes if the company is sold for cash.