Effect of estrogen plasma binding on sexual differentiation of the rat fetus.

alpha-Fetoprotein, the estradiol-binding plasma protein (EBP), binds estradiol but not R 2858 (11beta-methoxy-17-ethynyl-estradiol) specifically. R 2858 interferes more markedly than estradiol with the sexual differentiation of the male rat fetus following treatment of the mother during the final stages of gestation. Moreover, its tissular uptake is higher. These facts suggest that alpha-fetoprotein protects the fetus from the high circulating hormone concentrations present in the pregnant mother. The hormone, once transferred to the fetus, is retained in its vascular bed by EBP.