The expression of Mam-3 antigens in normal human endometrium and endometrial carcinomas was investigated employing the monoclonal antibodies 67D11 (anti Mam-3a), 115H10 (anti Mam-3b), and 115C2 (anti Mam-3c). Dewaxed sections of formalin-fixed, paraffin-embedded curettings of 9 proliferative-, 6 interval-, 13 secretory endometria, as well as 16 endometrial carcinomas were used. In normal cycling endometrium the Mam-3a, -3b and -3c antigens were detected at the apical membrane of surface and/or glandular epithelium only, but to a different degree. The Mam-3a antigen was only detected in the surface epithelium of 66% of proliferative and 33% of interval endometrium. The Mam-3b antigen was detected in surface and glandular epithelium of all normal endometria, but was most widespread in proliferative endometrium. The Mam-3c antigen was detected in surface epithelium of proliferative, and interval endometrium especially, and in glandular epithelium in 22% of endometria irrespective of the phase. Increased expression of Mam-3a and -3c antigens was recorded in most carcinomas. A loss of polarity of staining was seen in grade III carcinomas especially, staining of the cytoplasma was recorded in half of the carcinomas. The expression of Mam-3c antigen in carcinomas seemed to decline with increasing grade of anaplasia. All carcinomas expressed the Mam-3b antigen. The present results indicate that the Mam-3a and -3c antigens are tumor-associated antigens of endometrial carcinomas and that they might be useful as diagnostic tools in endometrial pathology.