The serums of rainbow trout, Amago salmon, crucian carp, and calf showed inhibitory effects on modori-inducing proteinase (MIP) purified from threadfin-bream muscle, while the inhibitory activities per ml serum were diverse among the different individuals of the same species. In addition, the inhibitory activity seemed to be also changeable according to the physiological condition in the same species. Taking into consideration the practical use and the high content of the inhibitory activity, calf serum was used for the further study of the MIP inhibitor(s). When calf serum was chromatographed on a column of Ultrogel AcA 34, three distinct MIP inhibitors were detected. The molecular weights of these inhibitors were estimated to be more than 500, 000, 58, 000, and less than 15, 000, respectively. The former two inhibitors were supposed to be α2-macroglobulin and α1-proteinase inhibitor, respectively, while the 15, 000-dalton inhibitor seemed to be specific for MIP as far as examined. These inhibitors lost their inhibitory activities by heating at 100¢C, but fairly stable around modori-occurring temperature, at 50-70¢C.
