Therapeutic use of IL-2 can generate antitumor immunity; however, a variety of different mechanisms have been reported. We injected IL-2 intratumorally (it) at different stages of growth, using our unique murine model of mesothelioma (AE17; and AE17 transfected with secretory OVA (AE17-sOVA)), and systematically analyzed real-time events as they occurred in vivo. The majority of mice with small tumors when treatment commenced displayed complete tumor regression, remained tumor free for> 2 mo, and survived rechallenge with AE17 tumor cells. However, mice with large tumors at the start of treatment failed to respond. Timing experiments showed that IL-2-mediated responses were dependent upon tumor size, not on the duration of disease. Although it IL-2 did not alter tumor Ag presentation in draining lymph nodes, it did enhance a previously primed, endogenous, tumor-specific in vivo CTL response that coincided with regressing tumors. Both CD4+ and CD8+ cells were required for IL-2-mediated tumor eradication, because IL-2 therapy failed in CD4+-depleted, CD8+-depleted, and both CD4+-and CD8+-depleted C57BL/6J animals. Tumor-infiltrating CD8+ T cells, but not CD4+ T cells, increased in association with a marked reduction in tumor-associated vascularity. Destruction of blood vessels required CD8+ T cells, because this did not occur in nude mice or in CD8+-depleted C57BL/6J mice. These results show that repeated doses of it (but not systemic) IL-2 mediates tumor regression via an enhanced endogenous tumor-specific CTL response concomitant with reduced vasculature, thereby demonstrating a novel mechanism for IL-2 activity.