Immunologic techniques have proven to be useful tools in characterizing the action of pituitary protein hormones on the target organs. Specifically, an antibody to luteinizing hormone can prevent ovulation, delay implantation, or cause fetal resorption depending on the time of administration of the antiserum to female rats. The immunologic interference with ovarian function appears to be associated with the steroidogenesis process during early pregnancy, and with fetal placental function during mid pregnancy in the rat. Progesterone synthesis is diminished, probably by the prevention of luteinizing hormone from stimulating the Δ5β hydroxysteroid dehadrogenase within the interstitial tissue of the ovary. In later pregnancy, the antibody combines with specific cells within the placenta, thereby eliminating any placental influence on the ovarian steroidogenic process.