MILWAUKEE (AP) _ The parent company of Milwaukee-based Pabst Brewing Co. says it plans to challenge Adolph Coors Co.'s proposed purchase of the Stroh Brewery Co.

S&P Companies, based in Mill Valley, Calif., said Tuesday it would file a lawsuit later this week or early next week alleging the Coors' plan would violate federal antitrust laws, said William M. Bitting, S&Ps legal counsel.

''This combination is going to create another oligopoly that's very tough on the rest of us,'' Bitting said. ''It's anti-competitive.''

S&P, which was among bidders for Stroh, also will participate in a formal hearing before the antitrust division of the U.S. Justice Department the week of Oct. 9, Bitting said.

Coors, based in Golden, Colo., announced Monday it planned to purchase the majority of Stroh's assets for $425 million in a deal that analysts said could create a ''Big Three'' of brewing.

The top two brewing companies - Anheuser-Busch Inc., St. Louis, and the Miller Brewing Co., Milwaukee - control 65 percent of the domestic beer market.

Analysts estimated that a Coors-Stroh combination could grab about 19 percent of the market, based on current shares. Pabst is the nation's sixth largest brewer.

The deal, which Coors does not expect to complete until at least early 1990, requires U.S. Justice Department approval.