As soon as Ron Hill opened the can, he knew the mushrooms were bad.

He didn't know, though, how bad things really were: that the mushrooms were laced with a deadly strain of botulism, that the packer had shipped out more than 5 million cans from the same batch, that potentially poisonous mushrooms were waiting for consumers on warehouse and grocery store shelves in 30 states and the District of Columbia.

All that Ron Hill, a Rockford, Ill., fireman, knew when the mushrooms exploded from the can was that he'd better call the government. Within hours, inspectors had seized all similar cans from the grocery. Within three days, the Food and Drug Administration had confirmed the presence of Type B botulinum toxin, the most potent of food poisons. Consumer warnings were issued at once, and FDA investigators began a dragnet to track down millions of cans.

Today, seven weeks after Ron Hill opened the can, almost all the mushrooms have been retrieved, without a single confirmed case of botulism.

The system that worked so swiftly and smoothly in the case of the poisoned mushrooms was designed, staffed, and managed by one of the most maligned professional groups in the nation, a group that politicians have been attacking for decades: federal bureaucrats.

The bureaucrats who run the protective system at FDA -- an agency that celebrates its 75th birthday today -- manage about 950 produce "recalls" every year, tracking down food, drugs, cosmetics and medical devices that do not meet federal standards.

But eight or 10 times a year the FDA has to gear up for a "Class One" recall, aimed at an acute threat to human life. Among the more notorious Class One cases were the cranberry scare at Thanksgiving, 1959, and the Bon Vivant Vichyssoise recall (another botulism problem, with one fatality and one critical illness) in 1971.

The mushroom drama began in mid-April at the Kelton, Pa., packing plant of Oxford Royal Mushroom Products Inc., where a breakdown in processing procedures permitted botulinus bacteria to contaminate various batches of mushroom stems and pieces. Some of the tainted cans found their way to the Eagle Food Store in Rockford, where Ron Hill was shopping for a dinner he would cook for his fellow firefighters at Rockford's Station No. 6.

As soon as Hill's can opener pierced the lid, a "putrid" shower of mushroom pieces splatter the kitchen. He immediately called the Winnebago County Health Department, which sent out an investigator and notified the local office of the FDA.

"We sent [FDA investigator] Charlie Peterson right out to the store," said Arlyn Baumgarten, chief of the agency's Waukegan regional office. "And we knew right away it was going to be pretty bad, because about half the cans on the shelf were swelling up," a sign of contamination.

Baumgarten called the FDA's microbiology lab in Cincinnati to warn that immediate testing would be required. "Several suspect cans were sent by air express to the lab, and by Sunday afternoon there was a "presumptive" finding of botulism contamination.

Back in Waukegan, Baumgarten was frantically trying to figure out where the tainted cans had come from -- a difficult task, because the cans bore only the local grocer's house brand name. Eventually, he traced them back to a food warehouse in Peoria. From there, the trail led to a national food distributor. Following that clue finally pinpointed Oxford Royal as the source.

That discovery brought Paula Oliver, the director of investigations at FDA's Philadelphia office -- the closest to Oxford Royal -- into the case. Working with the packing firm, she deployed a small army of FDA investigators, state and local inspectors, and food industry officials who combed warehouses and food stores from coast to coast.

"What we had to do was find out how many cases were involved and where they had gone, which is not all that easy," Oliver said. "You're talking probably, in number of retail establishments, more than 100,000 we had to go to . . . . We found these cans in Alaska, Hawaii, some cases got to San Juan. I think some showed up at military bases overseas. And with botulism, that's the greatest potential danger of any contaminent, you don't have any time to waste.

"But it looks like we're going to get out of this one without a single case" of disease, she went on. "We were very lucky -- but we are also very experienced in using this system when we have to." If you Spot . . . BOTULISM

It occurs in canned low-acid vegetables like corn, beans, tomatoes, mushrooms. If you spot the warning signs -- swollen or leaking cans, four-smelling vegetables -- call the Food and Drug Administration (D.C. area office: 285-2578).