A total of 26 specimens of histologically determined, cutaneous Bowen's disease from 23 patients were investigated for the incidence of mutant p53 protein expression by an indirect immunoperoxidase technique using the monoclonal antiserum CM1 on paraffin-embedded formalin-fixed tissue sections. Its potential role as a label of cutaneous malignancy was also evaluated: 15 specimens (57.7%) showed nuclear positivity of dysplastic keratinocytes, whereas 11 specimens (42.3%) were immunonegative. Cytoplasmic labeling was never a feature. In general, labeling was predominantly diffuse in distribution although in a significant proportion it was focal. One of two concurrent basal cell carcinomas arising adjacent to plaques of Bowen's disease was immunopositive. Focal immunopositivity of "cytologically normal" adjacent epithelium was seen in six specimens (23%). The study confirms the presence of mutant p53 protein in the majority of nongenital Bowen's disease. Its identification within nuclei of the adjacent "normal" epidermis may represent a potential marker of early (visible) neoplastic transformation.