27 sera from normal male donors an equal number from female donors and 10 sera of women taking oral contraceptives (OC) were compared with 38 sera of pregnant women in the second half of pregnancy to determine t he incidence of pregnancy-associated glycoproteins (SP SP and SP ). SP was present in elevated amounts only in pregnancy sera. SP and SP were found in pregnancy sera in women taking OCs and in a variety of different diseases. The pregnancy-associated glycoproteins occur in trace amounts in almost all normal sera. These proteins were characterized as reactants of the acute phase and therefore designated AP-glycoproteins. Detection of AP might prove valuable in differential diagnosis assessing the activity of a disease process and the effectiveness of therapy. SP has a high binding affinity for estradiol and testosterone and seems identical with steroid binding beta-globulin.