We have evaluated the functional properties of the OKT8+/OKT4+ T-cell subpopulations in nine patients with a new syndrome of acquired immune deficiency (AIDS). Despite polyclonal hypergammaglobulinemia in the sera of these patients, their peripheral blood lymphocytes (PBL) produced negligible quantities of immunoglobulin (Ig) when cultured in vitro for 8 days in the presence of pokeweed mitogen (PWM). Patient B cells, however, synthesized normal quantities of immunoglobulin when cocultured with T cells from healthy donors, indicating preservation of B-cell function. Unfractionated PBL or T cells of patient origin mediated marked suppression of pokeweed mitogen-driven immunoglobulin production by T and B cells from healthy donors. The suppressive activity was contained within the population of T cells bearing the OKT8 antigen and was sensitive to in vitro irradiation. On a per-cell basis, patient OKT8+ cells appeared to have greater suppressive activity than normal control OKT8+ cells. In addition, OKT4+ cells from patients had less helper activity for induction of immunoglobulin synthesis than control OKT4+ cells. Increased T suppression and reduced T help are probably a consequence of one or more viral infections and may contribute to progressive immune deficiency and susceptibility to malignancy in patients with the acquired immuno deficiency syndrome. © 1983 Plenum Publishing Corporation.