To investigate B-cell function in acquired immune deficiency syndrome (AIDS), we immunized a group of patients with AIDS with pneumococcal polysaccharide (tetradecavalent) and protein (keyhole-limpet hemocyanin) antigens. Antibody responses were determined three to four weeks after immunization. Compared with controls, patients with AIDS had significantly lower geometric mean antibody levels to polysaccharide before and after immunization. Levels before and after immunization were frequently below a level thought to correlate with protection. Patients with AIDS also had a significant reduction in their primary antibody response to the protein antigens. The results of these studies indicate that AIDS is associated with an acquired B-cell as well as T-cell immunodeficiency. It is suggested that future studies should consider the evaluation of passive antibody in the prevention of infection and/or prevention of progressive immunodeficiency.