20 children who were chronic carriers of group A beta-hemolytic streptococci (GABHS) were treated with oral clindamycin. Surface tonsillar cultures were obtained prior to therapy and 2 weeks after termination of therapy. They were processed for aerobic and anaerobic microorganisms. Mixed aerobic and anaerobic flora were obtained from all cultures. Prior to therapy, the average yield was 9 isolates (5 aerobes and 4.1 anaerobes) per specimen; after completion of therapy, the average yield was 5.7 isolates (3 aerobes and 2.7 anaerobes). GABHS, Staphylococcus aureus, Bacteroides fragilis, and Bacteroides oralis were completely eliminated after clindamycin therapy and the number of isolates of Bacteroides melaninogenicus and Fusobacterium sp. was reduced. Beta lactamase production was detected prior to therapy in 21 isolates recovered from 16 tonsillar surfaces (80%). These included all isolates of S. aureus (8) and B. fragilis (2), 8 of 18 B. melaninogenicus (44%), and 3 of 5 B. oralis (60%). Only one isolate of beta-lactamase-producing strain of B. melaninogenicus was recovered after conclusion of therapy. Follow-up of the patients for 18 - 24 months (average 22 months) showed no recurrence of GABHS in 19 of the 20 children.