Oxidized protein hydrolase (OPH), an 80 kDa serine protease whose activity is inhibited by diisopropyl fluorophosphate (DFP), has been isolated from human erythrocytes [Fujino, T. et al. (1998) J. Biochem. 124, 1077-1085]. The presence of OPH in various biological samples was examined by enzyme-linked immunosorbent assay (ELISA) and immunoblotting using an anti-OPH antibody raised against OPH purified from human erythrocytes, and by [(3)H]DFP-labeling and successive SDS-PAGE/fluorography. Solubilized samples of human cell lines including K-562 cells, THP-1 cells and Jurkat cells, and rat tissues including brain, heart, liver, kidney, and testis, inhibited the anti-OPH antibody binding to OPH in ELISA. Immunoblotting of lysates of K-562 cells, THP-1 cells and Jurkat cells showed four immunoreactive protein bands including an 80 kDa protein. Immunoprecipitation of the [(3)H]DFP-labeled K-562 cell lysate and successive SDS-PAGE/fluorography showed the presence of only the 80 kDa DFP-reactive protein with OPH antigenic activity. The level of the 80 kDa immunoreactive protein in K-562 cells rose as the cells differentiated toward erythrocytes. Immunoblotting of human and rat plasma showed two immunoreactive protein bands, including the 80 kDa protein, and SDS-PAGE/fluorography of [(3)H]DFP-labeled rat and human plasma showed the presence of only the 80 kDa DFP-reactive protein. The results indicate that OPH is present in a wide variety of biological samples.