We have identified a novel human protein, PRC1, that is involved in cytokinesis. PRC1 is a goad substrate for several CDKs in vitro and is phosphorylated in vivo at sites that are phosphorylated by CDK in vitro, strongly suggesting that PRC1 is an in vivo CDK substrate, PRC1 has sequence homology to the budding yeast anaphase spindle Elongation factor Ase1p. Like Ase1p, PRC1 protein levels are high during S and G2/M and drop dramatically after cells exit: mitosis and enter G1. PRC1 is a nuclear protein in interphase, becomes associated with mitotic spindles in a highly dynamic manner during mitosis, end localizes to the cell midbody during cytokinesis. Microinjection of anti-PRC1 antibodies into HeLa cells blocked cellular cleavage, hut not nuclear division, indicating a functional role for PRC1 in the process of cytokinesis.