We used antibodies against alpha-actinin and myosin labeled directly with contrasting fluorochromes to localize these contractile proteins simultaneously in dividing chick embryo cells. During mitosis anti-alpha-actinin stains diffusely the entire cytoplasm including the mitotic spindle, while in the same cells intense antimyosin staining delineates the spindle. During cytokinesis both antibodies stain the cleavage furrow intensely, and until the midbody forms the two staining patterns in the same cell are identical at the resolution of the light microscope. Thereafter the anti-alpha-actinin staining of the furrow remains strong, but the antimyosin staining diminishes. These observations suggest that alpha-actinin participates along with actin and myosin in the membrane movements associated with cytokinesis.

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