Human fibroblasts were insensitive to tumor necrosis factor (TNF) alone but were killed when treated with TNF together with the inhibitor of protein synthesis cycloheximide. However, incubation with TNF before addition of cycloheximide protected these cells from cytotoxicity. This suggested that TNF induced some protective mechanism(s) and that protein synthesis was required for this induction. By labeling fibroblasts with [35S]methionine, we observed that TNF induced in these cells the synthesis of two proteins of Mr 36,000 and 42,000 (p36 and p42). This induction was dependent on the TNF concentration and was observed within a few hours after addition of TNF. Actinomycin D prevented synthesis of these proteins, suggesting that TNF induced transcription of the corresponding mRNAs. p42 appeared to be stable, whereas p36 turned over within a few hours. The physiological role of these proteins in the cellular response to TNF remains to be established.