The mixed lymphocyte-tumor cell reaction (MLTR) between peripheral lymphocytes and cryopreserved autochthonous leukemia cells was studied in 36 adult patients with acute leukemia after complete remission as a result of chemotherapy. In 20 patients the lymphocytes showed a significant blastogenic response to autochthonous leukemia cells. Eleven out of 18 patients with acute myeloblastic leukemia and 4 out of 5 with acute monocytic leukemia showed positive MLTR, whereas 5 out of 13 with acute lymphoblastic leukemia gave a positive reaction. Cryopreserved leukemia cells of the patients with not only positive but also negative MLTR were able to stimulate allogeneic lymphocytes, except in one case. The relationship between MLTR and the prognosis of leukemia was analyzed in 30 patients who were alive more than 6 months after diagnosis. No apparent correlation was observed between MLTR and the survival time from diagnosis.