Epidermal-type TGase (TGase 3) is involved in the formation of the cornified cell envelope by cross-linking a variety of structural proteins in the epidermis. Unknown proteases activate this enzyme from the zymogen form by limited proteolysis during epidermal differentiation. It has been difficult to isolate sufficient quantities of native enzymes from tissues for biochemical studies of the properties of TGase 3. In this paper, we circumvented these problems by expressing recombinant full-length mouse TGase 3 in a baculovirus system, and purifying it to homogeneity by successive chromatography and HPLC. Treatment of the purified recombinant protein with dispase, a bacterial protease known to activate zymogens, produced activated TGase 3. The migration of TGase 3 zymogen in SDS-polyacrylamide gel electrophoresis was anomalous when the proTGase 3 was pre-incubated with calcium ion. GTP inhibited the enzymatic activity of recombinant TGase 3. Calpain, a calcium-dependent neutral protease, was a candidate protease, but had no effect on the activation of TGase 3 zymogen.