Platelets contain a high concentration of the contractile proteins actin and myosin. The properties of these proteins are generally similar to their counterparts in muscle, suggesting that the platelet actin and myosin can generate contractile force. Both proteins are localized in the cytoplasm and little or none of either is present on the outer surface. The distribution of actin in the filopodia and myosin in the body of activated platelets suggests that the force for clot retraction is generated by retraction of the filopodia using a mechanism similar to that in muscle contraction.