Oncoprotein 18 (Op18) is a cytosolic protein that is expressed in all proliferating cells. This phosphoprotein is up-regulated in a variety of human neoplasm, and phosphorylation of its two Cdc2 kinase target sites, Ser-25 and Ser-38, fluctuates dramatically during the cell cycle. We have investigated the potential role of the Cdc2 kinase-mediated phosphorylation of these two sites by expressing a “Cdc2 kinase target site-deficient” mutant of Op18 (Op18-S25,38A), and analyzed the phenotype on the level of cell cycle regulation. The result shows that induced expression of Op18-S25,38A results in rapid accumulation of cells in the G2 phase of the cell cycle. The block in G2 seems transient, since prolonged incubation was found to result in a large fraction of the transfected cells entering S phase in the absence of mitosis, i.e. endoreduplication. In addition, a fraction (30%) of the transfected cells was blocked in mitosis. Whereas the morphology of the G2 arrested cells appeared normal, expression of Op18-S25,38A caused a serious defect during mitotic chromosome segregation. Analyses of the mechanism behind the phenotype of Op18-S25,38A suggest an essential role for Op18 during cell division and that the mutant interferes with the function of the endogenous gene product.