Earlier reports from this laboratory suggested that 10-nm filaments and microtubules act together in the movement and positioning of nuclei and centrioles. Sodium vanadate has been found to alter the distribution of 10-nm filaments and separate them from microtubules in virus-induced syncytia and uninfected cells. Accompanying this change in cytoskeletal elements in an alteration in the distribution of nuclei, centrioles, and other organelles. Nuclei in vanadate-treated syncytia were found in a circle or horseshoe arrangement, and 10-nm filaments were aggregated within the circle, whereas microtubules, were found in a network throughout the cytoplasm. Vanadate also caused a perinuclear aggregation of 10-nm filaments in single uninfected cells, whereas microtubules were throughout the cytoplasm, as in syncytia. Centrioles, mitochondria, rough endoplasmic reticulum, and lysosomes were scattered in the perinuclear area, with mitochondria and rough endoplasmic reticulum frequently closely associated, whereas the peripheral region of vanadate-treated cells contained ribosomes, microfilament bundles, and microtubules, but not 10-nm filaments. Vanadate limited virus-induced fusion of cells to polykaryocytes with 5--20 nuclei, in contrast to the massive syncytia found in untreated cells. These results indicate that vanadate separates 10-nm filaments and microtubules topologically and functionally, and support previous evidence that 10-nm filaments and microtubules act together in the movement and positioning of nuclei and other organelles.