The effects of estrogens and tamoxifen were analyzed on estrogen receptor-positive human breast cancer (MCF-7) transplanted into athymic nude mice. It was found that (1) the tumor growth and the proportion of 3H-thymidine-labeled cells were significantly increased in the 17 beta-estradiol dipropionate (E2) group, but significantly decreased in the tamoxifen (TAM) group with respect to the control group, and (2) the tumor content of insulin-like growth factor-1 (IGF-1) and the rate of IGF-1-positive cells were significantly lower in the E2 group, but significantly higher in the TAM group than in the control group. It was concluded that the tumor content of IGF-1 and the proportion of IGF-I-positive cells were inversely correlated to the tumor growth and the 3H-thymidine labeling index in vivo.