The Janus kinase Jak1 has been implicated in tumor formation by the Abelson oncogene. In this study we show that loss of Jak1 does not affect in vitro transformation by v-abl as defined by the ability to induce cytokine-independent B-cell colony formation or establishment of B-cell lines. However, Jak1-deficient, v-abl–transformed cell lines were more tumorgenic than wild-type cells when transplanted subcutaneously into severe combined immunodeficient (SCID) mice or injected intravenously into nude mice. Jak1 deficiency was associated with a loss in the ability of interferon-γ (IFN-γ)to induce growth arrest and/or apoptosis of v-abl–transformed pre-B cells or tumor growth in SCID mice. Moreover, IFN-γ mRNA could be detected in growing tumors, and tumor cells explanted from SCID mice had lost the ability to respond to IFN-γ in 9 of 20 cases, whereas the response to interferon-α (IFN-α) remained intact. Importantly, a similar increase in tumorgenicity was observed when IFN-γ–deficient cells were injected into SCID mice, identifying the tumor cell itself as the main source of IFN-γ. These findings demonstrate that Jak1, rather than promoting tumorgenesis as previously proposed, is critical in mediating an intrinsic IFN-γ–dependent tumor surveillance.