Autologous lymphocytes provide a potential vector for the delivery of a cytotoxic agent in patients with lymphoid cell malignancy. This report describes a phase I-II study using autologous lymphocytes to target the radionuclide indium-114m ((114m)In) in patients with refractory chronic lymphocytic leukaemia or small lymphocytic non-Hodgkin's lymphoma. Nineteen patients, the majority of whom had been heavily pretreated with conventional chemotherapy and radiotherapy, received between 69 and 211 MBq (114m)In-labelled autologous lymphocytes. Approximately 80% of the administered activity was localized in the liver and spleen, with around 5% accumulating in the bone marrow. Ten patients (53%) responded (one complete response and nine partial responses). The median duration of response was 7 months. The median survival for the responders was 14 months and for the non-responders was 3 months. The first notable response in every patient was a fall in peripheral lymphocyte count. The indium treatment was not associated with any subjective toxicity, although all patients suffered from myelosuppression, with thrombocytopenia being the dose-limiting factor. This study has demonstrated a significant anti-tumour effect in a group of patients with late-stage highly resistant disease.