Complete hematopoietic reconstitution using nonleukemic peripheral blood mononuclear cells has been achieved in animal models but not in humans. We treated two patients who had metastatic breast carcinoma involving the bone marrow and who had failed conventional therapy with high-dose chemotherapy and total body radiation. Cryopreserved autologous peripheral blood mononuclear cells (6.3-8.4 X 10(8)/kg patient weight) obtained by leukapheresis before high-dose therapy were returned to the patients intravenously. In one patient, evidence of bone marrow engraftment was present, but the patient died before full reconstitution of the peripheral blood cells occurred. Bone marrow engraftment and return of all cell lines to the peripheral blood occurred in the second patient. These findings demonstrate that human hematopoietic reconstitution can be achieved with autologous, peripheral blood, mononuclear cell transfusions following high-dose therapy. This approach may be useful to patients who have contraindications for a bone marrow harvest but who are otherwise candidates for autologous bone marrow transplantation.