Up to four antigenic constituents have been found in the plasma of pregnant women which are not detectable in specimens from normal untreated non-pregnant women, or from men. They are visualized by immunodiffusion methods with hyperimmune rabbit antiserum to pregnancy plasma, after exhaustive absorption with lyophilized normal untreated non-pregnant female plasma. These constituents are distinct from each other as judged by their differing electrophoretic mobilites. It is unlikely that they represent any of the known normal plasma proteins visualized by gel diffusion because of the absorption methods used. In addition, it has been shown that none of them represent human chorionic gonadotropin, human placental lactogen, oxytocin, C-reactive protein, oxytocinase (cystine-amino-peptidase), alkaline phosphatase or esterase, and they are unrelated to bovine prolactin. Preliminary data revealed that one of these pregnancy-associated plasma constituents is synthesized during the course of oral hormone contraception.