Immunohistochemical differentiation of basal cell epithelioma from cutaneous appendages using monoclonal anti-glycoprotein antibody TNKH1. Its application in Mohs' micrographic surgery. TNKH1, which was primarily developed to detect differentiated melanocytic tumor cells, was found to recognize basal keratinocytes of hair follicle and some basal keratinocytes of human epidermis. Thus, TNKH1 decorated the basal cells of following structures: epidermis (39 of 54, only part of each specimen [OPES]), upper hair follicle (one of 24, OPES), lower hair follicle (21 of 21, very high rate of each specimen [VHES]), sebaceous duct (14 of 15, VHES), sebaceous gland (ten of 14, germinative cells near duct), eccrine duct (three of 19, OPES). Epithelial tumors, considered to be derived from or differentiating toward hair follicle such as trichilemmoma (one of one, VHES) and basal cell epithelioma (BCE) (32 of 32, VHES) were labeled not only in the peripheral cells but in their entirety. On the other hand, epidermal tumors, such as seborrheic keratosis (ten of 11, OPES), actinic keratosis (two of three, OPES), and squamous cell carcinoma (one of two, OPES), showed an irregular peripheral basal cell staining as in normal epidermis. The apocrine sweat apparatus and eccrine secretory portion were negative. Eccrine ductal tumors such as syringoma (two tested), eccrine acrospiroma (one), and eccrine carcinoma (two) were TNKH1 negative. Taking advantage of this total labeling of BCE versus peripheral labeling of the hair follicle, the authors could distinguish BCE tissue from other structures clearly. Among confusing structures the upper hair follicle and the eccrine duct were excluded easily because of their negative staining with TNKH1. The lower hair follicle was TNKH1 positive but only in the outer basal layer, whereas the BCE was TNKH1 positive in its entire basaloid cells. The result indicated that TNKH1 will be a useful antibody in Mohs' micrographic surgery. 