We have recently identified a 6,449 bp cDNA, termed colonic, hepatic tumor over-expressed gene (ch-TOG), that is highly expressed in human tumors and brain. Its single open reading frame encodes a putative 218,000 Da polypeptide, TOGp. Antibodies generated against a bacterially expressed TOGp fragment specifically recognize a 218, 000 Da polypeptide in two human cell lines and in brain. Immunofluorescence microscopy using affinity-purified TOGp antibodies revealed that the distribution of TOGp was dependent upon the cell cycle. During interphase, TOGp was found concentrated in the perinuclear cytoplasm, where it co-localized with ER markers. In contrast anti-TOGp antibodies stained centrosomes and spindles in mitotic cells. TOGp co-sedimented with taxol-stabilized microtubules in vitro. Moreover, a TOGp enriched fraction promotes microtubule assembly both in solution and from nucleation centers. Finally, sequence comparison and immunologic cross-reaction suggest that TOGp is homologous to XMAP215, a previously described microtubule associated protein (MAP) from Xenopus eggs. These results suggest that TOGp is a MAP and that TOGp/XMAP215 may be necessary for microtubules rearrangements and spindle assembly in rapidly dividing cells.