Mitosis comprises a complex set of overlapping motile events, many of which involve microtubule-dependent motor enzymes1,2. Here we describe a new member of the kinesin superfamily. The protein was originally identified as a spindle antigen by the CHO1 monoclonal antibody3 and shown to be required for mitotic progression4,5. We have cloned the gene that encodes this antigen and found that its sequence contains a domain with strong sequence similarity to the motor domain of kinesin-like proteins. The product of this gene, expressed in bacteria, can cross-bridge antiparallel microtubules in vitro, and in the presence of Mg-ATP, microtubules slide over one another in a fashion reminiscent of microtubule movements during spindle elongation.