The aims of the study were to assess patterns of cell proliferation, using the monoclonal antibody MIB1, in a variety of non-neoplastic and neoplastic cervical squamous intraepithelial lesions. We also wished to determine whether this antibody is of value in the discrimination of different grades of cervical intraepithelial neoplasia (CIN) and in the distinction of CIN from non-neoplastic lesions that may histologically mimic CIN. Tissue sections of formalin-fixed, paraffin-embedded cervical colposcopic biopsies were stained immunohistochemically with MIB1 after microwave heating. Normal ectocervix and cervical specimens exhibiting immature squamous metaplasia, koilocytosis, CIN grades I, II, and III, and atrophic squamous epithelium were studied. In normal ectocervix, MIB1-positive cells were confined almost exclusively to the parabasal cell layer. Increased expression was seen in cases of koilocytosis and CIN, positive staining being present in progressively higher layers of epithelium in accordance with the degree of CIN. In immature squamous metaplasia, positive staining was generally confined to the parabasal layer with occasional scattered positive cells in higher layers of epithelium. There was little or no staining of atrophic squamous epithelium. The study confirms that proliferating cells, as demonstrated by MIB1 immunostaining, are found at progressively higher levels in cervical squamous epithelia from normal to koilocytosis to CIN grades I, II and III. Immunostaining with MIB1, which can be easily performed on routinely processed tissues, is of potential value as an adjunct to histology in the grading of CIN and the distinction of CIN from non-neoplastic lesions that may mimic CIN.