We evaluated if Medroxyprogesterone acetate (MPA) had an immunosuppressive action as progesterone and other steroid hormones. Lymphocyte function was evaluated by the blastogenic response to mitogens and by the E-rosette test. MPA was tested at 7 ng/ml and 100 ng/ml concentrations. Cells were either preincubated for 30 min., 90 min., 180 min. and 12 hr or incubated with MPA for 72 hr, 18 hr and 16 hr. MPA showed a significative inhibitory effect only when used at the concentration of 100 ng/ml.