Phase II was a multi-center randomized trial to evaluate the hazards of discontinuing daily oral penicillin at the age of five years. Within three months of their fifth birthdays, all children were randomized to continue oral penicillin prophylaxis or to stop prophylaxis. Each child was followed for a minimum of two years. The primary endpoint was a comparison of documented pneumococcal infection in children continuing penicillin after five years of age versus children whose prophylaxis was stopped at five years of age. Ancillary studies conducted in subsets of patients included: the prevalence of colonization of the nasopharynx with antibiotic resistant microorangisms; and the relationship of antibody response to pneumococcal vaccination to the incidence of pneumococcal sepsis in this patient population.