The reactivities of new monoclonal antibody 1C5 and anti-carcinoembryonic antigen (CEA) were determined immunohistochemically in 4 adenocarcinomas in situ, 20 invasive adenocarcinomas of various types, and 6 adenosquamous carcinomas of the uterine cervix, as well as in 10 endometrial adenocarcinomas and 10 normal cervices. Among the invasive adenocarcinomas, 90% were positively stained by 1C5 and 55% stained for CEA. Three of four in-situ adenocarcinomas were positively stained by 1C5 and two of four were positively stained by anti-CEA. All adenosquamous carcinomas were stained by 1C5 and four of six stained for CEA. Invasive adenocarcinomas always stained more intensely with 1C5 than did noninvasive lesions in the same specimen. Poorly differentiated adenocarcinomas were stained as strongly with 1C5 as were well-differentiated tumors, but CEA was less effective in identifying poorly differentiated lesions. 1C5 was also more useful than CEA was in distinguishing glandular from squamous neoplastic differentiation, and also appears to be useful in distinguishing endocervical from endometrial differentiation.