Human recombinant ER-60 was confirmed to have transglutaminase activity by a microtiter plate assay. Transglutaminase activity of ER-60 did not require calcium and was inhibited by cystamine, a substrate analogue. In addition, the transglutaminase activity of ER-60 was not inhibited by SH-blocking reagents. These results suggest that the properties of the transglutaminase activity of ER-60 are different from those in the cases of known mammalian transglutaminases of which the active site includes a cysteine residue.