Treatment of sensitive human myosarcoma cells (KYM-S) with exogenous tumor necrosis factor (r-TNF) resulted in the production of TNF by the cells. The newly synthesized cellular TNF was identified immunologically on Western blots and as a single 1.8-kilobase band on Northern blots. TNF synthesis began within 2 h of administration of the exogenous TNF in a dose-dependent manner. r-TNF also induced TNF synthesis in mouse tumorigenic fibroblasts (L-M). Resistant sublines of these cells as well as TNF nonsensitive human diploid fibroblasts possessed TNF mRNA without pretreatment, indicating an inverse correlation between levels of TNF expressed and sensitivity to the cytotoxic effects of exogenous TNF. It is conceivable that the newly synthesized cellular TNF functions in some protective manner to block cytolytic effects of exogenous TNF.