The inhibitory effects of TNFα on adipocyte differentiation are well described, however, the mechanisms are poorly understood. Early during hormonally-induced 3T3-L1 preadipocyte differentiation there is a requisite mitotic clonal expansion phase that is associated with significant regulation in p130 and p107 protein levels, two members of the retinoblastoma protein family that regulate cell cycle events through interactions with the E2F transcription factors. This regulation occurs within the first 24 hours of differentiation (Day 1) and is characterized by a transient increase in p107 protein and mRNA levels as well as a transient decrease in p130 protein levels. Here we describe that TNFα disrupts the normal pattern of expression of both p130 and p107 proteins, leading to a complete block in mitotic clonal expansion. Interestingly, TNFα-treated cells enter S-phase as determined by 5-bromo-2′-deoxyuridine uptake experiments, but rather than completing cell cycle, they are stimulated to undergo apoptosis.