Eglin C is an inhibitor of two serine proteinase-derived polymorphonuclear leucocytes (PMN) i.e., elastase and cathepsin G. Since the latter has recently been shown to be involved in the activation of platelets by stimulated PMN, the effects of recombinant eglin C in the PMN-platelet cooperation model were studied. First, the inhibitory capacity of eglin C against purified cathepsin G was measured spectrophotometrically by following hydrolysis of a specific synthetic substrate, N-succinyl-ala-ala-pro-phe-para-nitroanilide. The inhibition of the enzymatic activity of 180 nM (5 micrograms/ml) cathepsin G was directly proportional to eglin C concentration and reached 100% with 2 micrograms/ml (240 nM). Platelet activation generated by a submaximal concentration of cathepsin G (200 nM) was also totally suppressed by 2 micrograms/ml of eglin C. Inhibition was specific (a 100 times higher concentration of eglin C did not alter platelet activation induced by thrombin), and surmountable (an increase of cathepsin G concentration reduced the eglin C effect). Thus, the mechanism of inhibition by eglin C of cathepsin G-induced platelet activation could be explained by a stoichiometric relation between eglin C and cathepsin G as previously described. Investigations were then performed with the PMN-platelet cooperation model, using two distinct stimuli, N-formylmethionylleucylphenylalanine (FMLP) or recombinant human C5a, at submaximal concentrations, 2.10(-7) M and 10(-7) M, respectively. A concentration-dependent inhibition of platelet activation aggregation and serotonin release-lambda was observed. Eglin C used at 10 micrograms/ml and 25 micrograms/ml totally blocked the platelet responses induced by recombinant human C5a and FMLP, respectively. Leucotriene B4, but also thromboxane B2 production measured by radioimmunoassays, were observed under FMLP activation. In the presence of eglin C, thromboxane B2 formation was totally suppressed, whereas leucotriene B4 synthesis was still effective. In fact, the mechanism of inhibition of eglin C is located neither on PMN (leucotriene B4 formation by FMLP-activated PMN was not affected), nor on platelets (response to thrombin was unchanged). The target is most probably cathepsin G since eglin C suppressed thromboxane B2 formation by platelets challenged by this serine proteinase. These results constitute an argument in favor of the implication of cathepsin G in the PMN-mediated platelet activation. Moreover, they reinforce the hypothesis that this mechanism could be operating under in vivo pathologic conditions, since eglin C is capable of preventing or ameliorating some experimental pulmonary diseases.