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The lack of detectable tumor-specific cytotoxicity by the peripheral blood lymphocytes of patients with cancer may be due to a lack of cytotoxic lymphocytes or the presence of suppressor lymphocytes that inhibit cytotoxic cells. Unfractionated peripheral blood lymphocytes from 12 of 28 patients with osteogenic sarcoma were cytotoxic to osteogenic sarcoma cells in vitro (P less than 0,001). When the peripheral blood lymphocytes from patients whose lymphocytes were not cytotoxic underwent fractionation, a tumor-specific cytotoxic subpopulation was isolated from 11 of 13 patients (P less than 0.0001). Lymphocytes that inhibited cytotoxic activity of autologous tumor-specific cytotoxic lymphocytes were found in four of 10 patients with osteogenic sarcoma but not in six normal controls. Inhibitor lymphocytes form rosettes with sheep erythrocytes and adhere to nylon, whereas cytotoxic lymphocytes have a receptor for C3 but no surface immunoglobulin. The lack of tumor-specific lymphocytotoxicity in some patients can be due to inhibitor lymphocytes.