A novel serine proteinase inhibitor has been purified to homogeneity from the skeletal muscle of white croaker (Argyrosomus argentatus). The purification was carried out by ammonium sulfate fractionation, DEAE-Sephacel, heating treatment followed by column chromatographies on SP-Sepharose, Sephadex G-150 and gel-filtration high performance liquid chromatography, The molecular mass of the inhibitor was 55 kDa as estimated by SDS-PAGE and gel filtration. It specifically inhibited a myofibril-bound serine proteinase ((MBSP) isolated from the skeletal muscle of lizard fish (Saurida wanieso). No inhibition, however, was detected toward other serine proteinases such as bovine trypsin, bovine chymotrypsin and a myofibril-bound serine proteinase from carp (Cyprinus carpio) muscle. Interestingly, the sequences of tryptic digested peptide fragments of MBSPI revealed high identity to that of porcine phosphoglucose isomerase (PGI) (76%) and other PGIs. Furthermore, purified MBSPI exhibits PGI activity, suggesting the inhibitor is a protein closely related to PGI. When rabbit muscle PGI was investigated, it also specifically suppressed the activity of MBSP, It thus strongly suggests that MBSPI is actually PGI and conversely, PGI is a specific inhibitor toward myofibril-bound serine proteinase(s).