We reported the effects of serine protease inhibitors on the interaction of platelet cytoskeletal proteins. Serine protease inhibitors such as PMSF, FOY-007 or FUT-175 and aspirin inhibited the incorporation of myosin heavy chain into the 1% Triton X-100 insoluble residue during the aggregation induced by ADP. FUT-175 also suppressed the incorporation of actin binding protein and fibrinogen into the cytoskeletons. This may result in the inhibition of not only the second aggregation but also the primary wave of aggregation. Anti-hydrolytic activity of FUT-175 against trypsin was recovered in cytosol fraction at least 2×10-6M, when platelets were incubated with this agent, 10-4M. This seems to indicate that FUT-175 has unique site (s) of action in cytosol which is related to the inhibition of integration of cytoskeletons during the process of platelet aggregation besides it's action on arachidonate metabolism. In fact, zymogram of platelet cytosol fraction with Tosyl-L-lysine α-naphthyl ester as substrate, demonstrated 5 active bands (pl. 9.2, 8.1, 6.25, 5.9, 5.3, 4.4). Some of these cytosomal serine proteases might act as an important role in the formation of cytoskeletons during the process of platelet reaction.