We have previously implicated TNF-related apoptosis-inducing ligand (TRAIL) in innate immune surveillance against tumor development. In this study, we describe the use of TRAIL gene-targeted mice to demonstrate the key role of TRAIL in suppressing tumor initiation and metastasis. Liver and spleen mononuclear cells from TRAIL gene-targeted mice were devoid of TRAIL expression and TRAIL-mediated cytotoxicity. TRAIL gene-targeted mice were more susceptible to experimental and spontaneous tumor metastasis, and the immunotherapeutic value of α-galactosylceramide was diminished in TRAIL gene-targeted mice. TRAIL gene-targeted mice were also more sensitive to the chemical carcinogen methylcholanthrene. These results substantiated TRAIL as an important natural effector molecule used in the host defense against transformed cells.