Abstract The possible role of induced IL-1 in a number of in vivo actions of TNF was investigated. We were particularly interested to know whether a species-specific induction of IL-1 might explain the important differences observed between murine TNF and human TNF in systems such as lethal shock and the induction of long lasting, high levels of circulating IL-6 in mice. We also studied the possible involvement of IL-1 in the sensitization to TNF observed in tumor-bearing mice or in combination with D(+)-galactosamine or RU38486, particularly because such sensitization results in the loss of species-specific differences between both TNF. Using a specific rIL-1R antagonist (IL-1ra), which inhibits the binding of IL-1 to the IL-1R type I, we were able to protect mice against a lethal murine TNF injection. However, the induction of high levels of circulating IL-6 by murine TNF was not affected, although IL-1ra almost completely blocked the induction of IL-6 by exogenously administered IL-1. Furthermore, the increased susceptibility of tumor-bearing mice to human TNF, relative to control mice, or the sensitization by co-administration of RU38486 or D(+)-galactosamine do not seem to be mediated by IL-1.