Juvenile does were actively immunized against estradiol-17.beta., testosterone, progesterone or bovine serum albumin. Depending on the antigen, the antibody titers at 17 wk ranged from < 1:10,000-1:500,000. Specificity of the antisera was high as judged by cross-reactivity studies. Significant binding of [3H]estradiol-17.beta. (7-30%) was observed in 1:100 diluted sera raised against testosterone, and 21-48% binding of [3H]testosterone in sera raised against estradiol-17.beta. (binding in control animals being 1-8%). Total serum estradiol-17.beta. levels (radioimmunoassay) were not only elevated (P .ltoreq. 0.05, compared to controls) in does immunized with estradiol-17.beta. but also in does immunized with testosterone. Production of antibodies against progesterone did not interfere with receptivity but none of the does gave birth. Receptivity and consequently pregnancy were prevented by immunization against estradiol-17.beta., suggesting that estrogen is necessary for receptivity in the rabbit. Receptivity was also blocked in does immunized against testosterone. This is evidently a non-specific effect and a consequence of estradiol inactivation due to increased binding in serum.