The effectiveness of penicillin V, penicillin V potassium, benzathine penicillin G, erythromycin and tetracycline in eradicating beta-hemolytic streptococci from the pharynx of 410 infected patients or carriers was studied. Penicillin V and penicillin V potassium were administered in daily oral doses of 375 to 750 mg (600,000 and 1,200,000 units), respectively, for 10 days. The benzathine penicillin G was injected once intramuscularly in similar doses. The broad spectrum antibiotics were given in daily oral doses of 30 to 50 mg/kg, up to 1 gm, for 10 days. The bacterial cure rate was 84% for 224 patients treated with penicillin V and penicillin V potassium, and 86% for 129 patients treated with benzathine penicillin G in combined doses. Forty-eight percent of 23 patients treated with erythromycin and 38% of 34 patients treated with tetracycline were cured. The majority of the clinical bacterial relapses developed 14 to 28 days after the onset of treatment with penicillin V orally and 24 to 32 days after benzathine penicillin G, 375 mg intramuscularly. Approximately half of the patients with bacterial relapses had clinical manifestations of pharyngitis. Factors which might be responsible for the development of relapses and failures are discussed. Complications consisted of one case of otitis media and one case of glomerulonephritis, developing 12 and 22 days, respectively, after the onset of treatment. No case of rheumatic fever was observed. The data indicate that penicillin was definitely more effective than erythromycin and tetracycline in eradiating beta-hemolytic streptococci from the host, in the doses used. There was no significant difference in the effectiveness of penicillin V, penicillin V potassium and benzathine penicillin G. Attention is drawn to the relapse-and-failure rate of 15% in these patients despite penicillin therapy.