To improve our understanding of the roles of microtubule cross-linking motors in mitosis, we analyzed two sea urchin embryonic kinesin-related proteins. It is striking to note that both of these proteins behave as homotetramers, but one behaves as a more compact molecule than the other. These observations suggest that these two presumptive motors could cross-link microtubules into bundles with different spacing. Both motors localize to mitotic spindles, and antibody microinjection experiments suggest that they have mitotic functions. Thus, one of these kinesin-related proteins may cross-link spindle microtubules into loose bundles that are "tightened" by the other.