Morphological and biochemical studies suggest that actin in human platelets polymerizes from monomers or oligomers into long filaments more tightly structured in activated than in resting platelets. The polymerization and reorganization of actin filaments are regulated by cellular proteins. Profilin prevents actin polymerization by forming tight complexes with monomeric actin; gelsolin acts both severing filaments and inhibiting their elongation by capping at the barbed end; other actin-binding proteins nucleate polymerization and cross-link actin filaments into networks or bundles. The changes in the actin assembly state, which are under the control of calcium, seem essential for pseudopodal formation. Other platelet processes, such as granule centralization and contractile gel formation, are due to calcium-dependent actin-myosin interaction. In addition, Ca++ seems to inhibit through calmodulin the binding of caldesmon to actin, allowing actin linkage to myosin in a 'flip-flop' fashion.