Results of cephalexin monohydrate and potassium phenoxymethyl penicillin were compared in children with pharyngitis and group A streptococci in two consecutive studies, 1968 to 1969 and 1969 to 1970. Ampicillin anhydrous was included in the first study. The patients were assigned randomly to one of the antibiotics, 0.5 gm daily for ten days. Recurrences, detected by routine weekly cultures, of the original group A streptococcal types within three weeks after end of treatment were classified as failures. The failure rates in the first study were 11% (4/38) for cephalexin, 26% (10/39) for phenoxymethyl penicillin, and 27% (10/27) for ampicillin. The failure rates in the second study were 9% (4/46) for cephalexin and 20% (10/50) for potassium phenoxymethyl penicillin. In the combined studies cephalexin was more effective than potassium phenoxymethyl penicillin (P <.05).