The monoclonal antibody, INS-2, was raised against rat fibroblasts transformed by open reading frames E6 and E7 of human papillomavirus (HPV) DNA. In immunoperoxidase testing of frozen sections, the INS-2 antibody was reactive with all squamous cell carcinomas of the uterine cervix and esophagus tested. In contrast, no antibody binding was detected with adenocarcinomas of various origins. Similarly, normal tissues, lymphoid cells and erythrocytes from multiple donors were negative, except that binding localized at basal cells in normal squamous epithelium was observed. Interestingly, strong staining was observed in dysplastic cells of cervical intraepithelial neoplasia and at the growing edge of squamous cell carcinomas. The antigen for the INS-2 antibody is a non-sialyl glycoprotein with Mw. 40,000 and appears to be a squamous cell-specific cell differentiation marker, although it is not related to HPV-DNA-derived protein.