We have demonstrated that tumor irradiation enhanced the therapeutic effect of interleukin 2 (IL-2) on pulmonary metastases from a murine renal adenocarcinoma, Renca. To investigate the mechanism of interaction between tumor irradiation and IL-2 therapy, we have histologically evaluated the effects of each therapy alone or in combination on Renca pulmonary metastases. Following treatment of established lung metastases with irradiation and IL-2 therapy, lung sections were processed for H&E or immunohistochemical staining. We found that tumor irradiation or IL-2 therapy locally induced vascular damage, resulting in multifocal hemorrhages and mononuclear cell mobilization in the lung tissue. This effect was amplified in lungs treated with the combined therapy. Immunohistochemistry showed that irradiation produced a macrophage influx into irradiated tumor nodules, and systemic IL-2 therapy induced T-cell infiltration in tumor nodules. Lungs treated with the combined therapy exhibited massive macrophage, T-cell, and natural killer cell mobilization in disintegrating tumor nodules and in the lung tissue. This combined therapy caused a decrease in the number of proliferating tumor cells and an increase in the number of apoptotic cells, which were more marked than with either therapy alone. We suggest that the macrophages mobilized by radiation-induced tissue injury could play a role in phagocytosis of apoptotic tumor cells, processing and presenting of tumor antigens for a systemic immune response activated by IL-2. Tumor destruction may result from the concomitant action of activated T cells, natural killer cells, and macrophages infiltrating the tumor nodules.