Using Pneumocystis carinii organisms propagated through three passages in embryonic chick epithelial lung cultures, specific antigens and antisera were prepared for use in counterimmunoelectrophoresis and indirect immunofluorescent antibody techniques. These methods proved to be specific and sensitive for the detection of P. carinii antigen and antibody, respectively, in sera, and were applied to the study of cancer patients with P. carinii pneumonitis (PCP), cancer patients without pneumonitis, and normal children. Antigenemia was detected in 95% of patients with PCP, in 15% of cancer patients without pneumonitis, and in none of the normal children tested. In cross-sectional and longitudinal studies of normal infants and children, acquisition of serum antibody to P. carinii was demonstrated to occur progressively with increase in age. By 4 years of age two thirds of the normal children were found to have antibody to P. carinii in titers of 1:16 or greater. These studies indicate that subclinical P. carinii infection is highly prevalent in normal children, analogous to other opportunistic infections where active disease is manifest predominantly in the compromised host.