We demonstrated previously that surface-active material potently suppresses early proliferative responses of lymphocytes to a wide variety of immune stimuli in vitro. It is now evident that in vivo, effector B and T lymphocytes can be recruited into lung parenchyma subsequent to their generation in extrapulmonary lymphoid tissues. The purpose of the present study was to examine the effects of surface-active material on proliferation, differentiation, and expression of effector functions of cytotoxic T cells and antibody-forming B cells in vitro in order to gain insight into the potential immune regulatory role of surface-active material in vivo. Normal spleen lymphocytes were cultured in vitro for 5 days with either allogeneic lymphocytes to generate cytotoxic T cells or with sheep erythrocytes to generate antibody-forming B cells. Surface-active material was added at various intervals after the cultures were initiated, and the effects of such additions on the subsequent proliferation, differentiation, and expression of cytotoxic T cells and antibody-forming cells were determined. Addition of surface-active material on days 0 through 3 suppressed both lymphocyte proliferation and the subsequent differentiation of effector lymphocytes. By contrast, addition of surface-active material after day 3 exerted no measurable effect on proliferation or on the generation of effector lymphocytes. We conclude that in vitro the immunosuppressive activity of surface-active material is exerted primarily during early proliferative phases of immune responses and that once these have occurred, surface-active material does not inhibit the later stages of differentiation and expression of effector cell functions. We speculate that in vivo, surface-active material may suppress local proliferation of lymphocytes resident in the lung in response to inhaled antigens; however, it may not interfere with effector functions of partially or fully differentiated B and T lymphocytes that are recruited into lungs from systemic sources.