Last summer, Stanley Lapidus's largest customer, General Motors Corp., handed him some news that made his coffee cup shake in his hand: GM was buying equity in several of his competitors. "The day after the announcement, I was concerned," says Lapidus, founder of Itran Corp. "I kept wondering, 'What will this do to my business?"

Certainly nothing good. When the world's second-largest industrial company roars through a neighborhood of small businesses, the ground shakes and rumbles. GM has agreed to make a multimillion-dollar investment in 5 small machine-vision companies, which make machines that are critical to quality control. The automated vision systems inspect the parts on an assembly line and will eventually be used to guide robots around the factory floor. GM's anointing of 5 from the more than 100 companies that are competing in machine vision -- among the best known is publicly traded Automatix Inc. -- has transformed a crowded market of struggling startups into a divided field of haves and have-nots.

All of those companies are battling for a fairly modest market that will probably amount to only $150 million this year. The lure: Industry sales have doubled every year since 1980, and they could reach $1 billion by 1990. No doubt an industrywide shakeout was due sometime soon, but GM has laid a heavy foot on the accelerator.

The competitors, which figured that good products and marketing would carry the day, were in for a surprise: The marketplace has responded by doing a headstand. "The feeling is that if you don't have a big Daddy Warbucks to prop you up, there are going to be serious questions about your survival," says one observer. Laura Conigliaro, an analyst with Prudential-Bache Securities Inc., says that GM's precedent has "sent a lot of companies scurrying to find corporate partners." For an unanointed business like Itran, she adds, the best strategy probably lies in "becoming a good second source. It should be there in the hope that one of the five companies will stumble."

Of course, that could hardly be the role that Itran had in mind before GM made its move. One chief executive officer, whose company was shunned, claims that GM is giving purchasing priority to the five. And when vision technology matures at these five companies, GM may proclaim a standard that effectively freezes out other suppliers, or at least leaves them far behind in product development.

GM is not the first corporate titan to lasso a small company in its hour of need. Last fall, IBM Corp. made a similar move after its efforts to create voice and data communications systems foundered. When it acquired Rolm Corp., a leader in the field, IBM solved its own problems -- but, like GM, it left a badly shaken market behind. "What GM is saying," observes Richard D. Schwarz, a first vice-president with E.F. Hutton & Co., "is that it wants machine-vision technology on the factory floor, and it wants it sooner rather than later."

That is a welcome message for the small companies in whose pockets GM is stuffing cash. The rest can count on only one thing: With GM at the wheel, they are in for a rough ride.