Transglutaminases are a family of Ca2(+)-dependent enzymes that catalyse the formation of isopeptide bonds between the side chains of glutamine and lysine residues. The enzymes have been hypothesized to be involved in a wide range of cellular processes, including growth and differentiation and stabilization of the cytoskeleton. The human epidermal carcinoma-cell line, A431 cells, have relatively high amounts of a cytosolic transglutaminase activity that varies upon treatment of the cells with epidermal growth factor. We demonstrate here that this cytosolic activity has the biochemical and immunological properties of a tissue transglutaminase. We also report the purification of this enzyme to apparent homogeneity by a protocol which involves a novel affinity-elution step. Polyclonal antibodies to the transglutaminase were raised and used to identify the enzyme by Western blotting. The availability of purified transglutaminase and antitransglutaminase antibodies will permit further study of the role of this enzyme in the growth of this hormone-responsive human tumour-cell line.