The effect of pharmacologic doses of sex hormones on NK and ADCC activity against YAC-1 lymphoma and CRBC target cells was studied. Estradiol (E) and testosterone (T) administration for 2 weeks caused a substantial reduction of splenic NK activity in TA3 mice of either sex. In prolonging the treatment time, the intensity of suppression was gradually increased. Both E and T have apparently no inhibitory effect on ADCC activity of TA3 mice, although the ADCC activity slightly increased in the early stage of T treatment. The ADCC activity of T-treated male mice was slightly higher than that of E-treated males. Passive transfer of the splenic mononuclear cells and serum of treated mice does not affect the NK and ADCC activity of normal recipient mice. Addition of different concentrations of sex hormones into the culture medium or pre-treatment of effector cells for 12h failed to change the NK and ADCC activity of murine splenic cells.