Sarcoma - lymphocyte-mediated cytotoxicity and serum blocking of this activity was studied by the in vitro microcytotoxicity test in three patients with osteogenic sarcoma who received preoperative irradiation. All three patients had complete histologic disappearance of the tumor between three to five months after irradiation. Lymphocyte cytotoxicity was present before and after treatment in two patients and post-treatment in one. No serum blocking activity was detected in any of these three patients. They are all alive over four years post-therapy without evidence of active disease.