Four highly sensitive monoclonal antibodies were assessed for their use in an immunoperoxidase technic on cutaneous sections. Three, B2, C23, and 2A1, were found to discriminate basal cell carcinomas (BCCs) from benign proliferative disorders of skin, and 2A1, a monoclonal antibody against the heavy chain component of class 1 human leukocyte antigens (HLA) for the first time directly demonstrated the absence of this structure from the malignant cell membrane. However, the deeper portions of hair follicles were also found to be unlabeled, and further study of benign follicular tumors which may histologically mimic basal cell carcinomas is suggested.