A 64-year-old slaughterhouse worker with advanced non-Hodgkin's lymphoma developed septicemia and pneumonia. Mycoplasma arginini, a wall-free prokaryote found in a variety of domestic animal hosts, was repeatedly isolated from blood and bronchial washings from the patient. Immunosuppression, in part caused by hypogammaglobulinemia, probably played a key role in predisposing the patient to a fatal infection. This case suggests that animal mycoplasmas should be considered in the list of infectious agents acquired by immunosuppressed hosts.