Blood lymphocytes of patients with solid tumors were assayed for cytotoxicity against autologous and allogeneic primary tumor cells. The lymphocytes killed autologous tumor cells in 7 of 25 cases (28%) and allogeneic tumor cells in 2 of 37 tests (5%). Lymphocytes from healthy donors were rarely cytotoxic for the biopsy cells, which indicates that these cells have low natural kill sensitivity. The autoreactivity that may reflect the immunological recognition of tumor cells was not altered by pretreatment of the effectors with interferon (IF). In contrast, killing of allogeneic tumor biopsy cells was induced by IF in approximately 50% of tests, with the lymphocytes of both the tumor patients and the healthy donors. The mechanism of the alloreactivity is most likely a consequence of IF-induced polyclonal activation of cytotoxic potential and the lymphocytes that are committed to recognize the alloantigens expressed on the particular target manifest the killing function. When the biopsy cells were explanted and kept in culture for 5-6 d, their susceptibility for the lymphocyte damage increased, and they were killed by the IF-treated cells also in autologous combinations. Whether this change in sensitivity is a result of qualitative or quantitative changes in antigen expression or of other changes in the properties of the cell membrane is unknown.