PIP: To detect antigens in the plasma of pregnant women that were not found in nonpregnant untreated normal women or males, highly sensitive immunodiffusion techniques with hyperimmune rabbit antiserum were used. The number of pregnancy-associated plasma constituents increased as pregnancy progressed in the 165 patients studied, with all 4 constituents usually seen in the third trimester. The 60 males and 111 nonpregnant women studied did not show any of these antigens. There were significant differences between second and third trimester reactions. (p less than .001). None of the antigens represented human chorionic gonadotropin, human placental lactogen, oxytocin, C-reactive protein, oxytocinase, alkaline phosphatase, or esterase. One of these constituents is present during combined estrogen-progesterone therapy.