Twelve cooperative study of sickle cell disease centers and eleven other clinical institutions initiated the Penicillin Prophylaxis in Sickle Cell Disease (PROPS I) clinical trial in August 1983. The objective of the trial was to determine whether the regular daily administration of oral penicillin would reduce the incidence of documented infection due to Streptococcus pneumoniae in children aged 3 months to 3 years with sickle cell anemia. The trial was scheduled to end in February 1986 but was terminated eight months early, after an average of 15 months of follow-up, when an 84 percent reduction in the incidence of infection was observed in the penicillin group as compared to the placebo group.