The influence of danazol on the expression of lymphocyte-mediated cytotoxicity toward the stromal component of the endometrium was examined. Stromal cells were isolated from endometrial biopsy specimens of nine women without endometrial pathology and 12 patients with endometriosis. A cytotoxicity assay by 51Cr release was performed to determine the effect of danazol on natural killer lymphocyte lytic activity. A significant suppression of spontaneous cytotoxicity against stromal cell targets of both study subjects was observed when effectors were pretreated for 48 h with danazol at a concentration of 5 x 10(-6) mol/l. The time-course of the danazol effect indicated that drug activity was detectable as early as 24 h after the addition of the drug and increase up to 72 h. These findings confirm that danazol may influence the immune system at many levels. In particular, they demonstrate that the drug is also able to inhibit the natural immunity toward the stromal component of the endometrium.