131I-labeled monoclonal antibodies were used in therapeutic trials in two potentially useful clinical situations: disseminated melanoma (intravenously administered Fab fragments; 21 patients) and disseminated peritoneal adenocarcinomatosis (intraperitoneal injection of IgG; 5 patients). Acute toxicity observed is consistent with mild bone marrow suppression of acute radiation syndrome and the observed toxicity is dose related in a manner that conforms to the expected human response to total body irradiation. For single doses of both i.p. administered and intravenously administered 131I-labeled anti-tumor antibodies, 100 rad to red marrow, calculated by the absorbed dose fraction method (MIRD), appeared to be a threshold below which significant acute toxicity was unlikely.