The malignant intraepithelial proliferations--malignant melanoma level I, bowenoid epithelial dysplasia, and mammary as well as extramammary Paget's disease--may cause differential diagnostic difficulties. We have examined 12 cases of malignant melanoma level I, nine cases of bowenoid epithelial dysplasia, 17 cases of extramammary and five cases of mammary Paget's disease for S100 protein, carcinoembryonic antigen (CEA), cytokeratin, and keratin to evaluate the sensitivity and specificity of these reactions with regard to their differential diagnostic value. Antibodies against S100 reacted specifically with the tumor cells in intraepithelial malignant melanomas; antibodies against CEA reacted specifically with the tumor cells in Paget's disease; and cytokeratin and keratin antibodies reacted with the epithelial tumor cells in Paget's disease as well as in bowenoid epithelial dysplasia. However, only antibodies to CEA and keratin showed 100% sensitivity. We conclude that the investigated antibodies may be of differential diagnostic value in cases of intraepidermal neoplasias, but that a negative reaction does not exclude diagnosis of these diseases.