The effect of TNF-α on cell death in rat fetal brown adipocytes maintained in primary culture was determined. TNF-α inhibited proliferation and induced apoptosis in these cells. Most of the cells undergoing apoptosis after TNF-α treatment did not express PCNA, suggesting an induction of apoptosis by TNF-α in non-proliferative cells. IGF-I but not EGF prevented TNF-α-induced apoptosis.