text: Aprotinin inhibited platelet aggregation induced by thrombin (0.25 U.ml-1) with IC50 200 kIU.ml-1, and inhibited the rise of cytosolic free calcium concentration in platelets stimulated by thrombin (0.1 U.ml-1) in the absence and in the presence of Ca2+ 0.5 mmol.L-1 (IC50 117 and 50 kIU.ml-1, respectively), but had no effect on the amounts of actin and myosin heavy chain associated with cytoskeletons.