Three patients with relapsed non-Hodgkin's lymphomas were given high dose 131I-labeled LYM-1 murine anti-lymphoma antibody in a Phase I trial. Each high dose infusion (65 mCi/m2) was preceded one week earlier by a tracer dose from which plasma pharmacokinetics were studied and dosimetry calculations were performed to predict doses to critical organs for the larger doses. None of the patients had definitive imaging of known sites of lymphadenopathy following either the tracer or therapeutic doses. Two of the three patients had objective tumor regressions which lasted >16 weeks. One patient developed HAMA (human anti-mouse antibody) within five weeks and was not further exposed to LYM-1. Marrow suppression proved to be the greatest toxicity.