Mad1 is a Myc antagonist that heterodimerizes with Max and functions as a transcriptional repressor. We have studied the effects of Mad1 on cell growth, cell cycle distribution, and apoptosis using Mad1-inducible cell lines. Expression of Mad1 inhibited cell proliferation, S-phase entry, and colony formation, changes that were accompanied by a reduction in CDK2 activity. The inhibition of Mad1 on cell proliferation was potentiated by serum starvation and was paralleled by accumulation of cells in the G0/G1 and the G2 phases of the cell cycle. Mad1 also reduced apoptosis induced by serum withdrawal and by the cytostatic drug cisplatinum. The effects on both cell growth and apoptosis were dependent on the mSin3 interaction domain of Mad1, which is necessary for recruitment of histone deacetylases and corepressors, suggesting that transcriptional repression is mediating these functions. Taken together with the expression pattern of Mad1, these results suggest that Mad1 plays an important role during initiation of differentiation by inhibiting cell proliferation and blocking apoptosis.