Expression of heat shock proteins (HSP) is an adaptive response to cellular stress. Stress induces tumor necrosis factor (TNF)- production. In turn, TNF- induces HSP70 expression. However, osmotic stress or ultraviolet radiation activates TNF- receptor I (TNFR-I) in the absence of TNF-. We postulated that TNF- receptors are involved in the induction of HSP70 by cellular stress. Peritoneal M were isolated from wild-type (WT), TNF- knockout (KO), and TNFR (I or II) KO mice. Cells were cultured overnight and then heat stressed at 43 ± 0.5°C for 30 min followed by a 4-h recovery at 37°C. Cellular HSP70 expression was induced by heat stress or exposure to endotoxin [lipopolysaccharide (LPS)] as determined by immunoblotting. HSP70 expression induced by either heat or LPS was markedly decreased in TNFR-I KO M, whereas TNFR-II KO M exhibited HSP70 expression comparable to that in WT mice. Expression of HSP70 after heat stress in TNF- KO M was also similar to that in WT mice, suggesting that induction of HSP70 by TNFR-I occurs independently of TNF-. In addition, levels of steady-state HSP70 mRNA were similar by RT-PCR in WT and TNFR-I KO M despite differences in protein expression. Furthermore, the effect of TNFR-I appears to be cell specific, since HSP70 expression in splenocytes isolated from TNFR-I KO was similar to that in WT splenocytes. These studies demonstrate that TNFR-I is required for the synthesis of HSP70 in stressed M by a TNF-independent mechanism and support an intracellular role for TNFR-I.