Don Minnick of the Alexandria Mariners hadn't even faced his first minor league pitch when he was told by his coaches to sign a contract binding him, his picture and his signature to a baseball card producer. In return, he received $5.

"The coaches said, "Here, sign this,' and I did," Minnick said over the weekend. "I never collected baseball cards but you can be sure that I'll be on one soon," he said in a cocky tone.

Hundreds of minor league and major league players are signed to binding contracts with the Topps Chewing Gum Co. each year for $5 and the possibility of a guaranteed $250 plus royalties each year they play in the major leagues.

But Troops, the reigning bubble gum manufacturer and sole baseball card distributer, and the Major League Baseball Players Association are being sued by the number two bubble gum producer, the Fleer Corp., in an antitrust case just now being tried in U.S. District Court in Philadelphia.

Topps, which produces the ever-popular Bazooka brand of gum, and the Players Association say Fleer is off base with its charges of monopolistic practices.

To the major consumers in the markets - shown by Fleer experts to be elementary school pupils - just who controls the market is of little significance.

But to the lawyers and corporate executives involved in the four-year-old suit, just where the bubble bursts in this baseball card war could be of serious consequence to their profits and markets.

Last year alone, Topps grossed more than $9.2 million in baseball card sales. Fleer charges that because of Topps' exclusive rights with almost every major league player and a majority of minor league players, Fleer is losing a possible $1.5 million in baseball card competition each year.

Fleer packaged baseball cards with cookies in 1963, but the attempt to compete with Topps crumbled, according to Fleer President Donald Peck.

As a result of Fleer's inability to get into the market by signing enough players who already weren't contracted with Topps, it sold what few contracts it had to Topps in 1966 for $395,000. This left Topps with virtually all major league players under contract.

In 1968, Player Association President Marvin Miller renegotiated the players' contracts with Topps for greater benefits. Fleer sees this as a cooperative ploy by Topps and the association to shore up the market and prevent potential competitors from entering the market.

The month-old trial, overseen by federal Judge C. C. Newcomer, has included as an exhibit a mock candy store filled with sweets from each bubble-gum producer and testimony from expert baseball card analysts.

A final decision in this four-year-old suit could take weeks, possibly months, however, according to those familiar with the case.