Progestin (medroxyprogesterone acetate, trade name Depo-Provera, Upjohn, Belgium) in 6 body-weight-matched doses with those used in women, i.e., 0.05 mg/mouse, given intramuscularly at 4-5-day intervals (duration of one ovulation) induced a marked reduction of fertility with precocious atresia of Graafian follicles, minimal numbers of growing follicles and an excess of corpora lutea; the lymphoid system remained morphologically and functionally normal. A combination of progestin and androgen (testosterone isobutyrate, trade name Agovirin Depot Biotika, Czechoslovakia) in a total dose of 1 mg + 0.9 mg (body-weight-matched with that used in men) or one order higher and divided into 6 doses given at 17-day intervals, i.e. roughly one half of the duration of spermatogenesis, had no inhibitory effect on spermatogenesis, whereas higher doses damaged the lymphoid system. This damage was reflected in a markedly reduced thymus weight with great depletion of cortex lymphocytes and diminished capacity for antibody formation to sheep red blood cells. The significance of the immunosuppressive effect for estimating the risks involved in the administration of sex steroids in human contraception is discussed.