ABSTRACT: The unique pregnancy-associated (Pa) antigen, which is a class I antigen encoded by the major histocompatibility complex (MHC), elicits a nondestructive maternal antibody response. By contrast, the class I transplantation antigen RTI.Aa elicits a destructive antibody response in tissue transplantation but not during pregnancy. With the use of the avidin-biotin complex (ABC) immunohistochemical method, the Pa and RTl.Aa antigens were localized on the basophilic and giant cells of the basal zone trophoblast, the endovascular trophoblast and decidual interstitial trophoblast, and the chorioallantoic membrane but not on the labyrinthine zone trophoblast as early as the 12th day of gestation. These two antigens were also expressed on the epidermis, hair follicles, spleen, thymic medulla, bronchial epithelium, intestinal epithelium, the hepatic Kupffer cells, endocardium, endothelium of blood vessels, renal tubular cells and glomeruli, and renal pelvis and ureter of fetal and adult rat tissues. Absorption studies with placental tissue confirmed the presence of these two antigens in the rat placenta, and antibody-blocking studies confirmed their unique specificities.