B-lymphocyte dysfunction is a characteristic feature of the acquired immunodeficiency syndrome (AIDS) and of the AIDS-related complex. The aim of the present study was to further examine the influences exercised by the human immunodeficiency virus (HIV; formerly called human T-lymphotropic virus type III or lymphadenopathy-associated virus, HTLV-III/LAV) on normal human B lymphocytes. An unfractionated protein preparation, made from HIV purified by density gradient centrifugation, was previously shown to induce differentiation of normal human B lymphocytes into immunoglobulin-secreting cells. In the present analyses, this B-lymphocyte response peaked on day 6 or 7 after culture initiation and was found to be independent of the requirement for monocytes but to require T cells. Responses could also be elicited in cultures of purified B cells by the addition of T cells that had been exposed to HIV antigen. Inhibitors of protein synthesis (puromycin and cycloheximide) abrogated the responses. In contrast to its stimulatory effects, the same virus preparation was previously shown to inhibit polyclonal responses that are normally elicited in peripheral blood lymphocyte cultures by a T-dependent stimulus (pokeweed mitogen) and T-independent stimulus (Epstein-Barr virus). The present studies suggest that the inhibitory effects of the HIV antigen studied herein are targeted primarily at the B lymphocytes. The role of T lymphocytes in the HIV antigen-mediated inhibitory effects, although demonstrated, could not be conclusively established as an essential pathway. These findings elucidate mechanisms by which components of HIV exert stimulatory as well as inhibitory effects on human B lymphocytes and thereby lead to the dysfunction of these cells in HIV infection.