(FORTUNE Magazine) – When Paul Mullan went off to college in the early Sixties, his mother -- like moms everywhere -- threw away his baseball-card collection. But unlike most boys, Mullan, 45, didn't get cards out of his system. The result: He's now CEO of Fleer, one of America's top baseball card makers (and the inventor of bubble gum). Mullan, an industry veteran, worked for rival Donruss before helping to buy Fleer in an LBO in 1989. Since then he has shaken up this sleepy, family-owned Philadelphia company by switching its focus from hobbyists to the mass market. ''People were asking for Fleer products, and the company wasn't making them,'' he says. ''Card packs priced at 50 cents were going for $1.'' So Mullan raised output about 50% last year and sold more to big retailers like Wal-Mart and Kmart. For the first nine months of 1990, revenues jumped 83% to $119 million, and profits rose 298% to $19 million. Like kids getting a deal on a rare Mickey Mantle card, investors snapped up Fleer's stock at $15 a share when Mullan took the company public in June and drove its price as high as $30. In 18 months the $6 million he and his backers initially invested has mushroomed into $120 million, even with the stock price back down to $20. Mullan's challenge will be to maintain growth in a business where companies traditionally restrain production to enhance their cards' value. With newcomers crowding the infield, some analysts worry that Fleer has already flooded the market. Then again, maybe they're underestimating the ability of moms like Mrs. Mullan to take this oversupply problem into their own hands.