PI-6 and a 56-kD PI-6/proteinase complex are found in myelomonocytic cells. (A) HL60, THP-1, and U937 cells were lysed and equal amounts of protein were separated by 10% SDS-PAGE under reducing conditions and immunoblotted using polyclonal anti–PI-6 antisera or monoclonal anti–PI-6 antibody. (B) Monocytes, neutrophils, and lymphocytes were isolated from peripheral human blood as described in Materials and Methods. Cells were lysed in the presence of 1 μg/mL aprotinin, 150 μg/mL PMSF, 0.5 μmol/L leupeptin, and 1 μmol/L pepstatin to minimize degradation by plasma proteinases. Equal amounts of protein were separated by 12.5% SDS-PAGE under reducing conditions and immunoblotted using polyclonal anti–PI-6 antisera. Both gels contain 2 ng of recombinant PI-6 (rPI-6) loaded as a positive control. The lower band in the rPI-6 sample represents serpin cleaved in the reactive loop by nonspecific proteolysis during purification and storage.