A series of 23 punch biopsies proved to contain human papillomavirus (HPV) type 16 and with established clinical course (including HPV-NCIN, HPV-CIN I, and HPV-CIN II lesion), and 18 additional biopsies of HPV 6-, 11-, 16- or 18-induced genital lesions were analyzed immunohistochemically for expression of cytokeratin No. 19 polypeptide. An immunoperoxidase-ABC technique was used with a polyclonal antibody raised against a synthetic nonapeptide corresponding to the residues 2–10 of the NH2-end, non-alpha-helical region. This polyclonal cytokeratin No. 19 antibody stained mainly (but not exclusively) the basal cells of the normal exocervical epithelium (heterogeneous pattern). Basal cell staining was intense slightly more frequently in HPV-CIN than HPV-NCIN lesions, i.e., ++ or more in 14/24 (58.3%) versus 8/17 (47.0%), respectively. The difference was more marked in the staining of the superficial cells, 70.8 and 58.8% showing intense expression of cytokeratin No. 19, respectively. In 6 (21.4 %) of the 28 HPV 16 lesions, basal cell layer was intensely stained, as contrasted to none of the 13 HPV 6, 11 or 18 lesions. The most distinct feature was the well-defined granular staining pattern of the superficial layer in 8 out of 10 HPV 6/11 lesions, as contrasted to the homogeneous pattern in 24 out of 28 HPV-16-infected lesions. In superficial cells, regressed lesions exhibited intense staining in 9/13 (69.2%), as compared with only 4/10 (40%) of the progressed lesions. These deviations from the normal cytokeratin No. 19 expression pattern in cervical squamous epithelium might be associated with progression of the lesions from HPV-NCIN (usually of HPV 6/11 origin) into higher grades of HPV-CIN (usually caused by HPV 16).