AbstractLymphocytes from 24 patients with nephroblastoma and 24 controls have been tested for tumor‐related cytotoxicity in vitro. The micro‐plate technique has been applied in 26 tests and the cytotoxic plaque test in eight.In 10 out of 24 nephroblastoma patients with two autochthonous and eight allogeneic combinations, tumor‐related cell‐mediated cytotoxicity has been found. Lymphocytes from control subjects were less cytotoxic against nephroblastoma cells and affected tumor and control cells to the same degree. Lymphocytes from patients treated for primary tumors showed tumor‐destructive reactions more often than those from patients treated for disseminated disease.