We report a young child with hypoplastic anemia and antibody deficiency with normal serum immunoglobulin levels. Studies of antibody responses to bacteriophage phi X 174 revealed an amnestic response but was predominantly limited to IgM. A similar response was observed following booster tetanus toxoid immunization. Analysis of in vitro pokeweed mitogen-induced immunoglobulin synthesis by the patient's B cells suggested an intrinsic B-cell defect.