Human preovulatory follicular fluids (FF) obtained in the course of stimulated cycles were analyzed for their possible immunologic functions. Different concentrations of FF (20%, 2%, 1%) inhibited the mitogenic response of normal human lymphocytes to concanavalin A (Con A). Lymphocytes were assessed for immunosuppressor activity after preincubation with FF. Lymphocyte mitogenic response to Con A was only suppressed by cells preincubated with FF at concentrations of 2% and 1% for at least 48 hours. No evidence of suppressor cell induction was seen following incubation of lymphocytes with 20% FF, nor was any significant relationship between FF immunosuppressor activity and the outcome of in vitro fertilization observed. We conclude that some factor(s) in FF may be capable of directly inhibiting lymphocyte response and inducing immunosuppressor cell activity in vitro.