I reported previously (Higashi-Fujime, S., 1982, Cold Spring Harbor Symp. Quant. Biol., 46:69-75) that active movements of fibrils composed of F-actin and myosin filaments occurred after superprecipitation in the presence of ATP at low ionic strengths. When the concentration of MgCl2 in the medium used in the above experiment was raised to 20-26 mM, bundles of F-actin filaments, in addition to large precipitates, were formed spontaneously both during and after superprecipitation. Along these bundles, many myosin filaments were observed to slide unidirectionally and successively through the bundle, from one end to the other. The sliding of myosin filaments continued for approximately 1 h at room temperature at a mean rate of 6.0 micron/s, as long as ATP remained in the medium. By electron microscopy, it was found that most F-actin filaments decorated with heavy meromyosin pointed to the same direction in the bundle. Myosin filaments moved actively not only along the F-actin bundle but also in the medium. Such movement probably occurred along F-actin filaments that did not form the bundle but were dispersed in the medium, although dispersed F-actin filaments were not visible under the microscope. In this case, myosin filament could have moved in a reverse direction, changing from one F-actin filament to the other. These results suggested that the direction of movement of myosin filament, which has a bipolar structure and the potentiality to move in both directions, was determined by the polarity of F-actin filament in action.