Despite advances in the primary treatment of non-Hodgkin's lymphoma, relapse is common and treatment after relapse is unsatisfactory. Autologous bone marrow transplantation, although sometimes successful, has generally had disappointing results. We conducted a trial of such transplantation in patients with relapsed non-Hodgkin's lymphoma, using strict criteria in selecting patients; we included only those in whom disease was minimal after conventional treatment (nodal disease less than 2 cm and bone marrow involvement less than or equal to 5 percent on histologic examination) and whose tumor cells expressed the B1 antigen. Forty-nine patients meeting these criteria received cyclophosphamide and whole-body irradiation supported by transplantation of autologous bone marrow that had been treated in vitro with anti-B1 monoclonal antibody and complement. All patients had features of a poor prognosis, including relapse from primary chemotherapy, histologic conversion to more aggressive disease, and extra-nodal dissemination. Thirty-three patients had a history of bone marrow involvement--16 at the time that marrow was obtained. Hematologic and immunologic engraftment was achieved in all patients. Only two treatment-related deaths occurred, from venoocclusive disease of the liver and intracerebral hemorrhage, respectively. Disease-free remission without maintenance therapy has lasted from greater than 2 to greater than 52 months in 34 patients (median follow-up, greater than 11 months). These results are similar to those obtained in patients with advanced, high-grade non-Hodgkin's lymphoma treated with primary combination chemotherapy. This study demonstrates that autologous bone marrow transplantation has tolerable toxicity and high efficacy in a subset of patients who are otherwise incurable but still responsive to cytoreductive therapy. The results suggest a role for such transplantation in the treatment of selected patients with newly diagnosed non-Hodgkin's lymphoma.