Thirteen patients with relapsed or refractory Non-Hodgkin's Lymphoma were treated with 131I-Lym-1 during the course of a dose escalation trial. Principal aims were to establish the maximum tolerated single dose (MTD), as well as to assess clinical and dosimetric effects of the MTD. Patients were eligible if > 25% of tumor cells bound Lym-1 on immunohistochemistry, stain intensity was +2/4 or greater and human anti-mouse antibody (HAMA) assay was negative. Radioimmunotherapy was performed with escalating doses at levels of 50 mCi, 65 mCi/m2 and 80 mCi/m2 (50-139 mCi total). Patients were eligible for retreatment after 6-10 weeks if there was no severe toxicity, their disease was at least stable and HAMA remained negative. Three were retreated. Four have achieved partial responses which lasted 11, 11, 18 and 22 weeks. Acute toxicities included rigors (69%), fever (62%), nausea (46%), vomiting (46%), pruritus (23%), urticaria (23%), chest pain (23%) and bronchospasm (15%). HAMA developed in 3 patients. Myelosuppression, manifested as thrombocytopenia and neutropenia, was dose-limiting and defined the single dose MTD at 65 mCi/m2. Plasma radioactivity clearance was biphasic, with a 0.9 hr alpha-T1/2 and a 19.8 hr beta-T1/2. At completion of Lym-1 infusion, a mean of 45% of the injected dose was recoverable in the circulation. Images obtained within the first 2 hours indicated mean hepatic and splenic uptake was 29% and 11%, respectively. Radiation absorbed doses to tumor ranged from 18-61 rads; mean doses to whole body ranged from 17 to 71 rads.