Immunoprecipitation, SDS-PAGE, and Western blot of platelet lysates. Solubilized proteins contained in platelet lysates were subjected to immunoprecipitation with monoclonal anti-PCI-IgG as described in Materials and Methods. Precipitates were analyzed by SDS-PAGE (10% acrylamide) and immunoblotting using rabbit-anti-PCI-IgG. Bound antigen was detected with biotinylated-antirabbit-IgG followed by streptavidin-peroxidase and with TMB color substrate reagent. Purified urinary PCI was used as a control. In platelet lysates, only the 57-kD band corresponding to the active PCI was observed; bands corresponding to cleaved PCI were hardly seen.