Twelve patients with metastatic colorectal cancer participated in a Phase I trial of 131I-labeled chimeric B72.3 (human IgG4). Consecutive groups of patients received 18 mCi/m2, 27 mCi/m2 and 36 mCi/m2. No acute side effects related to antibody administration were noted. Bone marrow suppression was the only side effect; it was dose-dependent and correlated with whole-body radiation dose estimates. The lowest dose level produced no marrow suppression, whereas 27 mCi/m2 resulted in Grade 1 and 2 marrow suppression in two of three patients. The maximum tolerated dose was 36 mCi/m2 with all six patients at this dose level having at least Grade 1 and two patients with Grade 3 and 4 marrow suppression. Eight of 12 patients had radioimmune imaging of tumor sites at 5-22 days. Seven patients had an antibody response to initial infusion. On retreatment, whole-body kinetics and imaging were altered for patients with a high anti-ch-B72.3 response. Thus, chimeric B72.3 (IgG4) has limited utility as a means of delivering multiple therapeutic doses of 131I in the majority of patients; alternative strategies including second generation anti-TAG-72 monoclonal antibodies, other radioisotopes and other chimeric human isotypes will need to be pursued.