Microtubules are dynamic polymers that move stochastically between periods of growth and shrinkage, a property known as dynamic instability. Here, to investigate the mechanisms regulating microtubule dynamics in Xenopus egg extracts, we have cloned the complementary DNA encoding the microtubule-associated protein XMAP215 and investigated the function of the XMAP215 protein. Immunodepletion of XMAP215 indicated that it is a major microtubule-stabilizing factor in Xenopus egg extracts. During interphase, XMAP215 stabilizes microtubules primarily by opposing the activity of the destabilizing factor XKCM1, a member of the kinesin superfamily. These results indicate that microtubule dynamics in Xenopus egg extracts are regulated by a balance between a stabilizing factor, XMAP215, and a destabilizing factor, XKCM1.