The effects of neutrophil cathepsin G on the glycoprotein (GP) Ib-IX complex of washed platelets were examined. Cathepsin G resulted in a concentration- and time-dependent decrease in the platelet surface GPIb- IX complex, as determined by flow cytometry, binding of exogenous von Willebrand factor (vWF) in the presence of ristocetin, and ristocetin- induced platelet agglutination. Cathepsin G resulted in proteolysis of the vWF binding site on GPIb alpha (defined by monoclonal antibody [MoAb] 6D1), as determined by increased supernatant glycocalicin fragment (a proteolytic product of GPIb alpha); decreased total platelet content of GPIb; and lack of effect of either cytochalasin B (an inhibitor of actin polymerization), prostaglandin I2 (an inhibitor of platelet activation), or prior fixation of the platelets. However, cathepsin G resulted in minimal decreases in the binding to fixed platelets of MoAbs TM60 (directed against the thrombin binding site on GPIb alpha) and WM23 (directed against the macroglycopeptide portion of GPIb alpha). In contrast to its proteolytic effect on GPIb alpha, the cathepsin G-induced decrease in platelet surface GPIX and the remnant of the GPIb-IX complex (defined by MoAbs FMC25 and AK1) was via a cytoskeletal-mediated redistribution, as determined by lack of change in the total platelet content of GPIX and the GPIb-IX complex; complete inhibition by cytochalasin B, prostaglandin I2, and prior fixation of platelets. Experiments with Serratia protease-treated and Bernard- Soulier platelets showed that neither platelet surface GPIb nor cathepsin G-induced proteolysis of GPIb were required for the cathepsin G-induced redistribution of the remnant of the GPIb-IX complex or the cathepsin G-induced increase in platelet surface P-selectin. In summary, neutrophil cathepsin G modulates the platelet surface expression of the GPIb-IX complex both by proteolysis of the vWF binding site on GPIb alpha and by a cytoskeletal-mediated redistribution of the remainder of the complex. Prior studies show that, although thrombospondin 1, antiserine proteases, and plasma are all inhibitors of cathepsin G, the effects of cathepsin G on platelets, including an increase in surface GPIIb-IIIa, occur during close contact between neutrophils and platelets in a protective microenvironment (eg, thrombosis and local inflammation).(ABSTRACT TRUNCATED AT 400 WORDS).