An extensive outbreak of staphylococcal food poisoning occurred in Kansai district in Japan. As many as 13,420 cases frequently ingested dairy products manufactured by a factory in Osaka City. The main ingredient of these dairy products was powdered skim milk manufactured by a factory in Hokkaido. Staphylococcal enterotoxin A (SEA) (< or = 0.38 ng/ml) was detected in low-fat milk and approx. 3.7 ng/g in powdered skim milk. The total intake of SEA per capita was estimated mostly at approx. 20-100 ng. The assumed attack rate was considerably lower than those reported in previous outbreaks. SEA exposed at least twice to pasteurization at 130 degrees C for 4 or 2 s retained both immunological and biological activities, although it had been partially inactivated. The present outbreak was unusual in that the thermal processes had destroyed staphylococci in milk but SEA had retained enough activity to cause intoxication.