The value of intramuscular benzathine penicillin (Bicillin) for the prophylaxis of streptococcal skin infections was examined in a population in which this problem has been endemic for a number of years. Seventy-eight children from 18 families were enrolled in this controlled pilot study and received in a double-blind fashion a single intramuscular injection of either Bicillin or placebo followed in 6 weeks by the opposite of the first injection. Dosage of Bicillin was 600,000 units in children 6 years of age or younger and 1.2 million units in those 7 years or older. During the immediate weeks after injection the prevalence and incidence of skin lesions were reduced in children who had received Bicillin when compared to those who had received a placebo injection. In the 6 week follow-up period after each injection, the overall frequency of lesions was reduced 38 per cent after Bicillin administration, and the duration of protection in individual children ranged from 3 to 6 weeks (mean interval of 4 weeks). However, 18 per cent of children experienced a breakthrough within 5 weeks; the majority of these were 6 years of age or younger. Less protection in younger children may have reflected an influence of age or inadequacy of dosage of the Bicillin.