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The aim of this study was to evaluate the safety, toxicity and therapeutic response of non-myeloablative radioimmunotherapy using 131I-rituximab in previously heavily treated patients with B-cell non-Hodgkin's lymphoma (B-NHL).Nine patients with relapsed, refractory or transformed B-NHL received ten radioimmunotherapies. Patients had a median of 5 (range 2-7) prior standard therapies. Four patients had received prior high-dose chemotherapy followed by autologous stem cell transplantation, and eight had received prior rituximab therapy. Histopathology consisted of four mantle cell, one follicular and four diffuse large B-cell lymphomas. Rituximab, a monoclonal chimeric anti-CD20 antibody (IDEC-C2B8), was labelled with 131I using the Iodogen method. The administered activity (2,200+/-600 MBq) was based on a dosimetrically calculated 45 cGy total-body radiation dose. All patients received an infusion of 2.5 mg/kg of rituximab prior to administration of the radiopharmaceutical.No acute adverse effects were observed after the administration of 131I-rituximab. Radioimmunotherapy was safe in our patient group and achieved one complete response ongoing at 14 months and two partial responses progressing at 12 and 13 months after treatment. One partial responder was re-treated with radioimmunotherapy and achieved an additional progression-free interval of 7 months. Four non-responders with bulky disease died 4.8+/-2.0 months after therapy. Three patients had an elevated serum lactate dehydrogenase (LDH) level prior to radioimmunotherapy and none of the patients responded. Of two patients who received radioimmunotherapy as an additional treatment after salvage chemotherapy, one continues to be disease-free at 9 months and one relapsed at 5 months' follow-up. Reversible grade 3 or 4 haematological toxicity occurred in seven of nine patients. Median nadirs were 35 days for platelets, 44 days for leucocytes and 57 days for erythrocytes.Radioimmunotherapy with 131I-rituximab in previously heavily treated B-NHL patients was safe and well tolerated, and four out of ten therapies induced responses. Radioimmunotherapy was less efficient in patients with bulky disease and elevated LDH. Severe haematological toxicity in seven patients did not cause significant clinical problems. Radioimmunotherapy seems to be an additional therapeutic option in carefully selected therapy-refractory B-NHL patients.