The purpose of this study was to compare the frequency of recovery of aerobic and anaerobic organisms with interfering capability against group A beta-haemolytic streptococci (GABHS) and beta-lactamase-producing bacteria (BLPB) from the tonsils of GABHS carriers and non-carriers. The presence of aerobic and anaerobic bacteria capable of such interference in vitro was evaluated in cultures obtained from the tonsils of 20 healthy children who were non-GABHS carriers and 20 who were GABHS carriers, and also from 20 children who were asymptomatic after completing a course of penicillin for acute GABHS pharyngo-tonsillitis (PT) and were non-GABHS carriers and 20 who were GABHS carriers. In healthy children, 32 interfering isolates were recovered from 16 non-GABHS carriers (1.6 per child) and 13 were isolated from 7 GABHS carriers (0.65 per child) (P<0.001). In children who had suffered acute GABHS PT, 26 interfering organisms were recovered from 15 non-GABHS carriers (1.3 per child) and 8 were isolated from 5 GABHS carriers (0.4 per child) (P<0.005). In healthy children, 13 BLPB were recovered from 5 non-GABHS carriers and 13 were isolated from 6 GABHS carriers. In children who had suffered acute GABHS PT, 14 BLPB were recovered from 5 (25 %) non-GABHS carriers and 32 were isolated from 17 (85 %) GABHS carriers (P<0.05). It was demonstrated in this study that there was a higher rate of recovery of aerobic and anaerobic organisms capable of interfering with GABHS in non-GABHS carriers than in GABHS carriers. This was observed in all GABHS non-carriers and included healthy children as well as those recently treated for symptomatic GABHS PT with penicillin that failed to eradicate GABHS. A higher rate of recovery of BLPB was observed only in GABHS carriers who were treated with penicillin for GABHS PT.