Chinese hamster ovary cells in suspension culture, synchronized by thymidine blockade and release, were infected with Mengovirus at varying time intervals up to and including mitosis. The time course of virus production and virus yield per cell are similar in synchronized and randomly growing CHO cells. Virus added to premitotic cells precludes both cell division and formation of mitotic figures, whereas cells in mitosis at the time of virus addition continue to divide. Results indicate that Mengovirus infection results in the inhibition of the synthesis of macromolecules necessary for cell division and are consistent with previous observations that virus infection results in inhibition of cell RNA and protein synthesis. Measurements of the time lag between virus addition and termination of cell division further suggest that the synthesis of all macromolecules essential for cell division is not completed until 0.66 ± 0.13 hour before division.