Chinese hamster ovary cells (CHO) were arrested with hydoxyurea at the beginning of DNA synthesis. Subsequent treatment with caffeine induced cells to bypass S-phase and undergo mitosis with unreplicated genomes (MUG). Treated cells built a normal spindle and distributed unattached kinetochores to daughter cells. To determine if MUG cells obey the metaphase checkpoint, we used immunoflourescence to detect and localize known metaphase checkpoint and motor proteins. In addition, the drug taxol was used to stabilize microtubules in MUG cells. The localization of CENP- E, the presence of anaphase A, taxol arrest, and taxol release acted in a similar manner as in controls. The localization of kinesin differed from the controls and that of MAD2 was inconclusive. These results imply that MUG kinetochores behave similarly to controls and probably have an operational metaphase checkpoint.