The specific immune response against the malignant cells was investigated in patients with urinary bladder or larynx cancer. Lymphocytes from lymph nodes that drain the tumor site were tested for their proliferative and cytotoxic capacities against autologous malignant cells isolated from the primary tumor. In no occasion was a proliferative or a cytotoxic response observed. However, when the lymph node cell suspensions were depleted of cells expressing both OKM1 and Leu-7 markers by rosetting with the appropriate mAbs, a proliferative response could be observed. The lymphocytes responded to autologous tumor cells only if IL-2 was added to the cultures. IL-2 alone induced some cell proliferation, which was not, however, comparable to that observed in response to both IL-2 and tumor cells. A panel of allogeneic tumor cells consistently failed to stimulate OKM1-, Leu-7- cells in vitro. Response to autologous tumor cells was not caused by HLA-encoded molecules, as occurs in the autologous mixed lymphocyte reaction, since OKM1-, Leu-7- cells failed to be stimulated by autologous non-T cells. A proliferative response was observed only with cells from lymph nodes that had been classified as invaded by malignant cells according to histopathologic criteria. Cells from noninvaded lymph nodes consistently failed to respond. Cells stimulated with autologous tumor cells could be expanded in short-term lines by continuous addition of IL-2 and malignant cells. One of these lines, which comprised mainly T8+ cells, was stimulated to proliferate only by autologous tumor cells, and its proliferative response was inhibitable by anti-class I and not by anti-class II mAbs. This line showed lytic capacities against autologous malignant targets, while it was inefficient against all of the other allogeneic cells tested. In another set of experiments, the mechanisms whereby exogenous IL-2 had to be added to the cultures to sustain a proliferative response against neoplastic cells were investigated. When cocultured with autologous malignant cells, OKM1-, Leu-7- lymphocytes expressed IL-2 receptors, as could be assessed by anti-Tac fluorescent staining. Under these culture conditions, these cells did not produce IL-2, and no proliferation was observed. Addition of purified IL-1 to the cultures induced IL-2 production and cell proliferation. It is concluded that metastatic lymph nodes contain a T cell population that can be detected in a proliferative assay when both suppressor cells are removed and the appropriate molecular signals are supplied.