Scallop hemocytes contain a transglutaminase (TGase) that is electrophoretically different from the TGase in the adductor muscle. The optimum temperature of the hemocyte TGase was lower (about 15 degrees C), compared with the muscle TGase (35-40 degrees C). Other properties, such as the high sodium chloride (NaCl) and CaCl2 concentrations required for activation, instability in salt solutions, and the Km values against monodansylcadaverine (MDC) and succinylated casein, were similar for both enzymes. When hemocyte homogenate was incubated with MDC at 10 degrees C, MDC was incorporated into the 230 k and 100 k proteins of the hemocytes. The 100 k protein was only detected in the supernatant, the 230 k protein was insoluble, and the 210 k protein was detected in both fractions. In the absence of MDC, the 230 k, 210 k, and 100 k proteins were cross-linked by endogenous transglutaminase. The 230 k protein was most quickly cross-linked and formed huge polymers within 5 min. These results suggest that if scallop tissues are injured, hemocyte transglutaminase may be activated, initially cross-linking the insoluble hemocyte 230 k protein, followed by the 210 k and 100 k proteins, to form a cross-linked protein matrix with inter cross-linking of hemocyte sheets, to stop the bleeding.