The predominant form of the cross-linking enzyme, transglutaminase, in cultured normal human epidermal keratinocytes, is found in cell particulate material and can be solubilized by nonionic detergent. It elutes as a single peak upon either anion-exchange or gelfiltration chromatography. Monoclonal antibodies raised to the particulate enzyme cross-react with one of two transglutaminases in the cell cytosol. The second cytosolic transglutaminase, which has distinct kinetic and physical properties from the first, does not cross-react and is not essential for formation of the keratinocyte cross-linked envelope in vitro. The antitransglutaminase antibodies stain the more differentiated layers of epidermis in a pattern similar to that given by anti-involucrin antiserum. These observations support the hypothesis that the transglutaminase so identified is involved in cross-linked envelope formation in viva.