The objectives of this contract are to 1) identify interference by penicillin prophylaxis on the acquisition of antibodies to S. pneumoniae; 2) assess the emergence of penicillin resistant S. pneumoniae; 3) upon cessation of penicillin prophylaxis, compare the incidence of documented bacterial infection in that patient population with children receiving penicillin; 4) to determine if continued penicillin prophylaxis beyond 5 years of age interferes with the serum antibody response to booster immunization with the pneumococcal vaccine or subsequent development of natural serum antibodies to S. pneumoniae; 5) to determine, in children with S. pneumoniae infection, if they had detectable serum antibody to the infecting serotype; and 6) to determine if patients who develop severe pneumococcal infection have lower serum antibody titers to S. pneumoniae before infection or respond suboptimally to the pneumococcal vaccine as compared to patients without infection.