These studies are concerned with the isolation and characterization of the proteins associated with the cytoskeletal structure of human blood platelets. Two proteins, F-actin and actin-binding protein (ABP) have been isolated from platelets by a simplified procedure and when these are reassociated they form a structure resembling that which exists in the native platelet. Studies ar in progress to characterize the factors which may be involved in regulating the assembly-disassembly of the cytoskeleton of platelets.