Antibodies to a cytoplasmic myosin, rat lymphoma myosin, and to rat skeletal myosin were prepared in rabbits and shown to be specific for their corresponding antigens. The two antibodies did not cross-react. The skeletal myosin antibody was directly labeled with rhodamine, and the cytoplasmic myosin antibody was detected by indirect immunofluorescence with fluorescein-labeled goat anti-rabbit antibody. The two antibodies were used to examine developing rat muscle cultures for the presence and location of the antigens. The antibody to cytoplasmic myosin reacted with multinucleated myotubes and with all the mononucleated cells in the culture. The antibody to skeletal myosin reacted with myotubes and with a small fraction of the mononucleated cells. In the myotubes, the cytoplasmic myosin appeared to be localized primarily in two structures: fine stress fibers, often visible also by phase microscopy and present predominantly in the ends of the cells, and in a submembranous rim all along the cell's border. In addition, a diffuse fluorescence within the cells was observed. The skeletal myosin was localized in the central part of the myotubes in sarcomeres or in fibers without periodicities and was excluded from the ends of the myotubes. When the same cells were doubly stained with the two antibodies, the complementary distribution of the two isozymes was very clear. There was also a narrow region of overlap of staining, with cytoplasmic myosin present in some stress fibers that appeared to be continuous with fibrous elements containing skeletal myosin. Myotubes that rounded up with cytochalasin B or with trypsin displayed a diffuse distribution of both isozymes. When these cells were allowed to respread into extended configurations, the location of the two myosins were essentially the same as in untreated cells. The ability of myotubes to adhere to the surface and to move in culture may be related to the presence of cytoplasmic myosin. Our results show that in myotubes and myoblasts the two isozymes differ sufficiently to be localized in distinct regions of the cell and to be sorted out into different structures, even after the cytoplasmic contents have been reshuffled. The cell can, by some unknown mechanism, distinguish the two myosins.

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