Antisera raised against the soluble antigens of the endometrium of early pregnancy detected two antigenic proteins of \g=a\1 and \ g=a\ 2 mobility in extracts of this tissue and were termed antigens A and B. Neither antigen was detected in pregnancy sera or extracts of proliferative endometrium, but antigen B was detected in extracts of secretory endometrium and both were present in amniotic fluid and medium from i n\ x=r eq-\ vitro incubations of pregnancy endometrium. Fractionation of

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... elled medium on ion-exchange chromatography demonstrated that antigens A and B co-eluted with the proteins from which EP14 and EP15 were derived and which were the major secretory polypeptides of pregnancy endometrium in vitro. Further biochemical purification revealed that EP14 (Mr 32 000) was derived from a protein of native molecular weight 36 000 which existed in two forms, whereas EP15 (Mr 28 000) was derived from a dimeric glycoprotein of native molecular weight 56 000. Immunochemical studies demonstrated that antigens A and B are identical to these two secretory proteins and have been termed pregnancy-associated endometrial \g=a\1-and \g=a\2-globulins (\g=a\1and \g=a\2-PEG).