The 56-kD PI-6/proteinase complex contains a serine proteinase and is formed after lysis. U937 cells were lysed in the presence of 1 μg/mL aprotinin, 150 μg/mL PMSF, 0.5 μmol/L leupeptin, 1 μmol/L pepstatin, 100 μmol/L EDTA, or a mixture of all these inhibitors. Samples were incubated at 37°C for 10 minutes, separated by 10% SDS-PAGE, and immunoblotted using polyclonal anti–PI-6 antisera.