The death domain of tumor necrosis factor (TNF) receptor-1 (TNFR1) triggers distinct signaling pathways leading to apoptosis and NF-kappa B activation through its interaction with the death domain protein TRADD. Here, we show that TRADD interacts strongly with RIP, another death domain protein that was shown previously to associate with Fas antigen. We also show that RIP is a serine-threonine kinase that is recruited by TRADD to TNFR1 in a TNF-dependent process. Overexpression of the intact RIP protein induces both NF-kappa B activation and apoptosis. However, expression of the death domain of RIP Induces apoptosis, but potently inhibits NF-kappa B activation by TNF. These results suggest that distinct domains of RIP participate in the TNF signaling cascades leading to apoptosis and NF-kappa B activation.