A food-borne outbreak of group A β hemolytic streptococcus (GAS) pharyngitis occurred in an Israeli airforce base bet,veen 15 and 18 April, 1992. An epidemiologic investigation was conducted in a random sample of the base personnel. The effectiveness of prophylactic administration of penicillin to healthy individuals during the outbreak in preventing secondary spread of the disease was evaluated. 197 patients with pharyngitis were seen at the base clinic during the outbreak. The epidemiologic investigation indicated that the outbreak was food-borne. Consumption of processed white cheese that had been prepared without using a proper hand washing technique, 24 h before lunch on 15 April 1992, and subsequently kept at room temperature for 5 h, was significantly associated with GAS infection of the epidemic strain (p < 0.05). The food handler who processed the cheese had anterior cervical lymphadenopathy and GAS T type 8/25/Imp19 (the epidemic type) was found in his throat culture. The secondary respiratory attack rate among the healthy base personnel (n > 1,000) was 1.6%. 40 base personnel were treated by penicillin prophylaxis and had a secondary attack rate of O%. The medical personnel who were on duty during the outbreak had a secondary attack rate of 75% (p < 0.001). The use of penicillin prophylaxis did not have an advantage in preventing secondary respiratory spread of streptococcal pharyngitis. Penicillin prophylaxis might reduce the high secondary attack rate of the disease in risk groups such as the medical personnel.