Immunohistochemical staining for involucrin, a cytoplasmic protein synthesized during squamous maturation, was assessed in histologic sections from hysterectomy and cone biopsy specimens from patients with cervical neoplasia. In normal and condylomatous squamous epithelium, diffuse cytoplasmic staining was seen in the suprabasal layers, with no staining of the basal cells. Staining was absent in two cases of cervical intraepithelial neoplasia (CIN), grade III, in which the lesions were composed entirely of undifferentiated cells and markedly decreased in cases involving large numbers of basal cells. In 19 of 23 cases (83 per cent) of CIN, however, focal staining for involucrin was seen in large differentiated cells in the more superficial layers, and in two cases of keratinized CIN diffuse suprabasal staining was observed. Similarly, strong staining for involucrin was present in differentiated areas in one case of microinvasive squamous cell carcinoma and in 93 per cent of cases of infiltrating squamous cell carcinoma. These findings suggest that involucrin is a marker for maturation in cervical squamous epithelial neoplasms. Patterns of immunohistochemical staining for involucrin in keratinized dysplasia and differentiated squamous carcinomas should be taken into consideration if loss of involucrin staining is used as a criterion for neoplastic transformation of cervical epithelium, as has been proposed.