In addition to their well-established roles at the G1-S checkpoint, recent reports support a role for universal cyclin-dependent kinase (CDK) inhibitors in the control of G2-M and suggest that their induction may stimulate the occurrence of endomitosis or polyploidy in a number of physiological settings. In this report, the stable expression of the p120E4F transcription factor, which attenuates G1-S progression by elevating p21WAF1 and p27KIP1 protein levels, was shown to also interfere with the regulation of G2-M and cytokinesis. Exponentially growing cultures of p120E4F-expressing fibroblast cell lines had reduced levels of CDC2 kinase activity, elevated levels of Cyclin B1 protein, and continuously generated a subpopulation of tetraploid cells and elevated numbers of multinucleated cells. Coexpression of activated Ras, which stimulates Cyclin D1 expression and G1-S-specific cyclin-CDK kinase activities, alleviated these effects without reducing p21WAF1 or p27KIP1 protein levels; p120E4F/ras-expressing cell lines contained reduced levels of Cyclin B1 protein, a restoration of Cyclin B-CDC2 kinase activity to control levels, and exhibited no increase of tetraploid or multinucleated cells. Interestingly, changes in the expression of Cyclin B1 and, to a lesser extent, CDC2 were primarily regulated by post-transcriptional mechanisms. The results indicate that mechanisms which moderately elevate CDK inhibitor levels can reduce CDC2 kinase activity to the point of impeding normal G2-M function and suggest that two molecular determinants commonly associated with the induction of polyploidy in a number of tissues, i.e., elevated levels of universal CDK inhibitors and sustained CDK2 kinase activity, may be solely sufficient to initiate endomitosis.