More than 500 specimens of lung tissue were examined for Pneumocystis. Of the 38 infections detected, most were in immunodeficient patients. Samples of serum from approximately 600 healthy normal subjects and 117 children with acute lymphatic leukemia were examined by an indirect fluorescent antibody test. The age-related data from the normal children suggested that nearly 100% of children are infected with Pneumocystis during the first two years of life. Groups of patients with leukemia who had symptoms of pneumocystis pneumonia had significantly higher titers of IgG antibody than groups of patients with leukemia who did not have clinical symptoms and normal subjects. Nevertheless, the diagnostic value of the indirect fluorescent antibody test is limited, but serologic follow-up study can be useful. Groups of children with leukemia had lower mean titers of IgM antibody regardless of their clinical condition.