Patients with multiple myeloma are at increased risk of severe bacterial infection. A variety of immune deficits has been described in such patients, including a decreased primary antibody response and defects in complement and granulocyte function. The depressed humoral response appears to result primarily from the activity of suppressor monocytes. Pneumovax (Merck Sharp & Dohme, West Point, Penn) should be administered to patients with myeloma, although its effectiveness in this population has not been proven. The role of other potential modalities of treatment and prophylaxis, such as IV gamma globulin, requires further study.