To examine the cellular immune defect that predisposes patients with the acquired immunodeficiency syndrome (AIDS) to opportunistic infections, we tested T lymphocytes from 16 patients for the capacity to secrete macrophage-activating products (lymphokines) including gamma interferon. Mononuclear cells from 10 of 11 patients did not generate an effective lymphokine in response to mitogen, and 11 of 16 produced subnormal levels of gamma interferon (less than 300 U per milliliter). In addition, upon stimulation with specific microbial antigen, cells from none of 14 patients generated active lymphokines, and cells from 13 to 14 completely failed to secrete gamma interferon. However, the antimicrobial function of monocytes from the patients was intact, and once stimulated with normal lymphokines or gamma interferon alone, macrophages derived from patients' monocytes responded with enhanced and effective intracellular antimicrobial activity. These results suggest that impaired lymphokine production may predispose patients with AIDS to opportunistic infections, and they provide a rationale for using gamma interferon as immunotherapy.