5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is a small molecule in the flavanoid class that has antitumor activity thought to be due to ability to induce high local levels of tumor necrosis factor (TNF)-alpha that disrupt established blood vessels within tumors. The drug has completed phase 1 testing in humans and is currently in phase 2 trials in combination with chemotherapy. Although characterized as a "vascular disrupting agent," there are some studies suggesting that DMXAA also has effects on the immune system that are important for its efficacy. The goal of this study was to carefully define the immune effects of DMXAA in a series of murine lung cancer and mesothelioma cell lines with varying immunologic characteristics. We show that DMXAA efficiently activated tumor-associated macrophages to release a variety of immunostimulatory cytokines and chemokines, including TNF-alpha; IFN-inducible protein-10; interleukin-6; macrophage inflammatory protein-2; monocyte chemotactic protein-1; and regulated on activation, normal T-cell expressed, and secreted. DMXAA treatment was highly effective in both small and large flank tumors. Animals cured of tumors by DMXAA generated a systemic memory response and were resistant to tumor cell rechallenge. DMXAA treatment led to initial tumor infiltration with macrophages that was followed by an influx of CD8(+) T cells. These CD8(+) T cells were required for antitumor efficacy because tumor inhibitory activity was lost in nude mice, mice depleted of CD8(+) T cells, and perforin knockout mice, but not in CD4(+) T-cell-depleted mice. These data show that activation of tumor-associated macrophages by DMXAA is an efficient way to generate a CD8(+) T-cell-dependent antitumor immune response even in animals with relatively nonimmunogenic tumors. Given these properties, DMXAA might also be useful in boosting other forms of immunotherapy.