Recombinant PI-6 and native cathepsin G form an SDS-stable complex in vitro. Recombinant PI-6 (2 ng) was mixed with 1 to 200 ng of native cathepsin G and incubated at 37°C for 10 minutes. Samples were resolved by 10% SDS-PAGE under reducing conditions and then immunoblotted with polyclonal anti–PI-6 antisera.