An infant who received multiple transfusions during the first few days of life for rhesus disease became ill with recurrent infections when 6 months old. Hepatitis, thrush, Candida dermatitis, otitis media, and disseminated Mycobacterium avium intracellulare infection occurred by 14 months of age. Immunological studies showed raised immunoglobulin levels, decreased mononuclear-cell responses to allogeneic cells and mitogen, and a decreased helper/suppressor cell ratio. It was determined that one of the blood donors, who was well at the time of blood donation, had died 17 months after with multiple opportunistic infections and acquired immunodeficiency. The clinical and laboratory findings in our patient suggest that he acquired a transmissible infectious agent from a blood transfusion, resulting in acquired immunodeficiency, and that this agent was not cytomegalovirus, Epstein-Barr virus, or hepatitis B virus.