CHICAGO (AP) _ The source of a suspected salmonella outbreak among nearly 300 conventioneers remained a mystery today as two dozen hit hardest by the food poisoning recovered in a hospital.

Eight-five people attending the annual True Value hardware stores convention were sickened Wednesday, after a group of 200 reported symptoms Monday, said Mike McCarthy, a spokesman for the Chicago Health Department.

''It would certainly seem to be a salmonella problem,'' he said. ''At this particular time, we have not determined what substance may have been the agent.''

Both groups were poisoned by the same tainted food despite the delay in reactions, McCarthy said.

''These people either had original symptoms that declined and then flared up again, or they did not go for examination or treatment originally,'' he said.

Ambulances carried the victims to hospitals Wednesday, said Randi Killian, spokeswoman for the Mid-America chapter of the American Red Cross. Twenty-four people were admitted at Grant Hospital and most were listed in fair condition, spokeswoman Pat Leck said.

Health officials inspected the kitchens at the Hyatt Regency-Chicago, where conventioneers shared a buffet dinner Sunday night, but no tainted food was found and kitchens were allowed to remain open.

Authorities also were checking into a meal served Monday at the headquarters of True Value's parent firm, Cotter & Co., McCarthy said.

Preliminary results of laboratory tests showed that 10 patients admitted at Grant earlier this week suffered from salmonella poisoning, said Ginny Sharp, another hospital spokeswoman.

Symptoms of the contamination by the salmonella bacteria include nausea, vomiting, diarrhea and fever. Sufferers must take care to avoid dehydration, McCarthy said.