Abstract: Anti-platelet antibodies are known to contribute to some types of thrombocytopenia. In this work we investigated anti-platelet antibodies with opposite influence upon activation and kinetics of platelet caspases. A rabbit anti-platelet antibody induced a profound thrombocytopenia, which was associated with an increase of microparticles in plasma and an activation of platelet caspases, as detected by the binding of a carboxyfluorescein-labeled fluoromethyl ketone probe (FAM-VAD-fmk). Furthermore, microparticles and thrombocytopenia were prevented by the injection of a caspase inhibitor ZVAD-fmk. In contrast, an anti-CD18 mAb (M18.2) induced a thrombocytosis, due to an increased platelet life-span and which was evident in wild-type (+/+), but not in CD18−/− or CD87−/−, mice indicating a requirement of these two surface molecules. Activation of caspases was decreased in platelets from mice injected with the M 18.2 mAb, as evidenced by a decreased binding of the VAD probe, detected by flow cytometry, or an increase in the level of pro-caspase-3, seen on Western blots. These observations indicate firstly, that anti-platelet antibodies can either promote or inhibit activation of platelet caspases, and secondly, that the activation of caspases regulates platelet life-span.