The membrane skeleton of the human erythrocyte consists of many short actin filaments that are multiply cross-linked by long, flexible spectrin molecules into a continuous network in the plane of the membrane. The mechanical properties expected for this spectrin-actin network can account for the tensile strength of the erythrocyte membrane and for the remarkable deformability of the cells, yet not for their characteristic biconcave shape. Recently, an authentic vertebrate myosin as well as a non-muscle form of tropomyosin have been identified and purified from erythrocytes. The myosin is present with respect to the actin in an amount comparable to actin-myosin ratios in other non-muscle cells, and there is enough tropomyosin to almost completely coat all of the short actin filaments in the membrane skeleton. The implications of these unexpected discoveries for the molecular organization of the cytoskeleton are discussed, and a mechanism is proposed by which myosin could interact with the membrane-associated actin filaments to influence erythrocyte shape and membrane properties.