Sequential studies of cellular and humoral immunity were conducted in an infant with the Wiskott-Aldrich syndrome prior to and after a splenectomy for uncontrollable hemorrhage. All measures of cellular immunity showed gradual improvement during the 8-month period after surgery. Serum isohemagglutinins, diphtheria and tetanus antibodies, and the percentage of immunoglobulin-bearing B cells did not change significantly from presplenectomy values. The serum IgE concentration declined from a high of 10,800 IU/ml at 1 month postsplenectomy to a low of 860 IU/ml at 5 months after surgery and the IgG concentration gradually decreased from a high of 1880 mg/dl presplenectomy to a low of 620 mg/dl 8 months later. The platelet count ranged from 64,000 to 206,000/mm3 for the first 6 months after splenectomy. It decreased precipitously 6.5 months after the operation; at the same time there was a marked rise in platelet-bound IgG antibody (PB-IgG). The PB-IgG declined rapidly following vincristine therapy and, after another rise, declined more gradually following steroid therapy.