Cultured human epidermal keratinocytes express a transglutaminase which appears in epidermis only during later stages of normal cell differentiation (Thacher and Rice, Cell 40685, 1985). Using a monoclonal antibody affinity column, the keratinocyte transglutaminase has been purified approximately 1000-fold from non-ionic detergent extracts of the particulate fraction of a squamous cell carcinoma line. The prominent 92-kilodalton band in the purified material is recognized on Western Blots by a monoclonal antibody which also can immunoprecipitate the active enzyme. Further analysis of the 92-kilodalton band shows that it is apparently not antigenically related to tissue transglutaminase, or to the cytoplasmic enzyme expressed in cultured keratinocytes which is immunologically cross-reactive and of identical molecular weight to tissue transglutaminase. In monkey palmar and plantar epidermal homogenates significant transglutaminase activity appears to be membrane bound and chromatographs on anion-exchange as the keratinocyte enzyme does. It can be precipitated, at least in part, with the aid of specific monoclonal antibodies directed to keratinocyte transglutaminase. As determined immunohistochemically, the most likely membrane location of the enzyme in cultured cells is the plasma membrane, consistent with keratinocyte transglutaminase function in cross-linked envelope formation. Thigh, neonatal foreskin, psoriatic epidermis, and monkey esophagus are compared as to patterns of staining for keratinocyte transglutaminase. Cultured epidermal cells and psoriatic epidermis both show precocious expression when compared to normal epidermis, demonstrating that keratinocyte transglutaminase expression can be substantially modified during epidermal regenerative maturation or hyperproliferation.