Protein kinase C (PKC) isoforms regulate many platelet responses in a still incompletely understood manner. Here we investigated the roles of PKC in the platelet reactions implicated in thrombus formation as follows: secretion aggregate formation and coagulation-stimulating activity, using inhibitors with proven activity in plasma. In human and mouse platelets, PKC regulated aggregation by mediating secretion and contributing to alphaIIbbeta3 activation. Strikingly, PKC suppressed Ca(2+) signal generation and Ca(2+)-dependent exposure of procoagulant phosphatidylserine. Furthermore, under coagulant conditions, PKC suppressed the thrombin-generating capacity of platelets. In flowing human and mouse blood, PKC contributed to platelet adhesion and controlled secretion-dependent thrombus formation, whereas it down-regulated Ca(2+) signaling and procoagulant activity. In murine platelets lacking G(q)alpha, where secretion reactions were reduced in comparison with wild type mice, PKC still positively regulated platelet aggregation and down-regulated procoagulant activity. We conclude that platelet PKC isoforms have a dual controlling role in thrombus formation as follows: (i) by mediating secretion and integrin activation required for platelet aggregation under flow, and (ii) by suppressing Ca(2+)-dependent phosphatidylserine exposure, and consequently thrombin generation and coagulation. This platelet signaling protein is the first one identified to balance the pro-aggregatory and procoagulant functions of thrombi.