We have used in vitro immunotherapy before autologous bone marrow transplantation for three patients with acute lymphoblastic leukemia (ALL). Bone marrow was removed during remission, and mononuclear cells were separated by density-step centrifugation on Ficoll-Hypaque. The cells from each patient were treated with a heteroantiserum and complement to eliminate leukemic cells and were cryopreserved. Following chemotherapy and total body irradiation, the treated marrows were thawed and infused. All the patients showed positive evidence of returning marrow function before death. One patient who survived 4 months showed no evidence of leukemia at post mortem, and marrow sections demonstrated active hematopoiesis of all cell lines.