Abstract
Actin has been shown to be present in the nucleoli, kinetochore and centriolar regions, and in the mitotic spindle of rat kangaroo cells which have been stained with fluorescently labeled heavy meromyosin. The actin in the spindle is confined to the fibers that connect the chromosomes with the centriolar region. Actin was not present in astral fibers, in the continuous spindle fibers that connect the poles, or in non-kinetochore regions of the chromosomes. The specific localization of actin in chromosomal spindle fibers suggests an actin-mvosin interaction as the force-producing mechanism for chromosomal movement.