To understand the molecular basis of microtubule-associated motility during mitosis1,2, the mechanochemical factors that generate the relevant motile force must be identified. Myosin, the ATPase that interacts with actin to produce the force for muscle contraction and other forms of cell motility 3, is believed to be involved in cytokinesis but not in mitosis 4-7. Dynein, the mechanochemical enzyme that drives microtubule sliding in eukaryotic cilia and flagella8,9, has been identified in the cytoplasm of sea urchin eggs10-19, but the evidence that it is involved in cytoplasmic microtubule-based motility (rather than serving as a precursor for embryonic cilia) is equivocal. Microtubule-associated ATPases have been prepared from other tissues (reviewed in ref. 12), but their role in cytoplasmic motility is also unknown. Recent work on axoplasmic transport, however, has led to the identification of a novel mechanochemical protein called kinesin20, which is thought to generate the force for moving vesicles along axonal microtubules20-27. These results suggest that kinesin may also be a mechanochemical factor for non-axoplasmic forms of microtubule-based motility, such as mitosis. We describe here the identification and isolation of a kinesin-like protein from the cytoplasm of sea urchin eggs. We present evidence that this protein is localized in the mitotic spindle, and propose that it may be a mechanochemical factor for some form of motility associated with the mitotic spindle.