The effects of corticosteroid given in vivo on human lymphocyte subpopulation function were investigated using an in vitro system of pokeweek mitogen-stimulated immunoglobulin production. Peripheral blood lymphocytes were obtained from normal volunteers before and 4 h after the intravenous administration of methylprednisolone. Unfractioned peripheral blood lymphocytes showed a consistent decrease (mean ≅ 50%) in immunoglobulin and total protein synthesis after steroid administration. Utilizing separated thymus-derived (T) and bone marrow-derived (B) lymphocyte fractions, the pathophysiology of this alteration in immunoglobulin production was elucidated. B lymphocytes obtained after steroid treatment showed a markedly diminished immunoglobulin response (20% of normal) to normal T lymphocytes and to normal T cells that had been irradiated to remove suppressor T lymphocyte function. All major classes of immunoglobulin (IgG, IgM, and IgA) were affected. T lymphocytes procured after steroid administration were capable of providing normal amounts of T cell help for B cells in immunoglobulin production. However, suppressor T lymphocyte activity, observed with normal T lymphocytes at high T to B cell ratios, was absent from the post-steroid T lymphocytes. This loss of suppressor T lymphocyte function was not due to the presence of excess help as irradiated pre- and poststeroid T cells provided equal amounts of helper activity. On recombining the poststeroid treatment B cells, which are hyporesponsive in immunoglobulin synthesis, with the posttreatment T lymphocytes, which lack suppressor activity, diminished amounts of immunoglobulin were produced which correlate well with the effects observed with unseparated cells. Thus, corticosteroids have differential effects on the lymphocyte populations involved in immunoglobulin biosynthesis. B cell responsiveness is diminished, suppressor T lymphocyte activity is removed, and helper T lymphocyte function is unaffected.