The distribution of A, B, and H blood group antigens was studied by means of peroxidase-anti-peroxidase technique in normal skin and in lesions of carcinomas in situ (solar keratoses, Bowen's disease), squamous cell carcinoma, keratoacanthomas, and verrucae. In normal skin, the epidermis of persons of blood group O showed H antigens throughout the epidermis; of blood group A, H and A antigens; and of blood group B, H and B antigens. In lesions of solar keratoses, there were no antigens of blood groups in the irregular downward proliferations. In five of 11 cases of Bowen's disease, there were no antigens of blood groups in the epidermis. In eight out of 10 cases of squamous cell carcinoma, no antigens of blood groups were found in the islands of the neoplastic process, but in two cases they were present in a patchy distribution. In the benign lesions examined, the antigens of A, B, and H blood groups were always present, although in verrucae the staining was confined to the upper layers of the epidermis only.