The effectiveness of piperacillin was investigated in 30 children operated upon for peritonitis: 13 had acute appendicitis with puriform peritoneal reaction, or a recently perforated appendix; 5 had generalized peritonitis of appendicular origin, and 13 had intraperitoneal abscess. In the 12 children who underwent right iliac appendicectomy (with post-operative drainage in 3), piperacillin was administered alone during 5 days; clinical and bacteriological cure was obtained in all cases; the mean duration of stay in hospital was 7 days. The 5 cases of generalized peritonitis required drainage; piperacillin was given alone in 4 of them and combined with an aminoglycoside and metronidazole in one who was in poor general condition. Bacteriology showed a predominance of Escherichia coli alone or associated with other organisms. Clinical and bacteriological cure was obtained in 3 patients; the mean duration of stay in hospital was 12 days. Seven of the 13 cases of intraperitoneal abscess needed drainage. Piperacillin was administered alone for 7 days on average in 10 cases and combined with an aminoglycoside and metronidazole in 2 cases. Eight patients had a favourable course, 5 developed complications. In all 3 groups piperacillin was tell tolerated. A patch of urticaria was noted in 2 cases and a transient skin rash in 2 other cases. No neutropenia was observed in these children whose treatment never exceeded 10 days.