METHODS In this randomized open study, 325 children aged two to 15 years with acute tonsillitis and a positive test of GA beta H streptococcal antigen were treated with josamycin 50 mg.kg-1.day-1 b.i.d for 5 days, or penicillin 50,000 to 100,000 IU/day t.i.d for 10 days. Clinical assessments and throat cultures for GA beta HS isolation were performed at the inclusion visit (V1), at the end of treatment visit (V2: day 12 for all patients) and at the follow-up visit (V3: day 30). In case of positive GA beta HS culture, the bacterial DNA by RFLP was performed to differentiate between the persistence (presence of original strain at V2), relapse (eradication at V2 and acquisition of same strain at V3) and reinfection (eradication at V2 and acquisition of different strain at V3). RESULTS Two hundred and twenty-three patients were included in the bacteriological and clinical criteria per protocol analysis. At V2, eradication rates were comparable: 82% in josamycin and 80% in penicillin patients; clinical cure rates were 90% and 89%. At V3, relapse of GAS assessed only on clinically and bacteriologically cured patients at V2 occurred in 12% of josamycin patients and 12.8% of penicillin patients. Tolerance was good; 14% and 10% of josamycin and penicillin patients respectively experienced an adverse event. CONCLUSIONS In this non-inferiority study, the efficacy of a 5-day course of josamycin is comparable to reference treatment in GA beta HS tonsillitis in children.