33 patients with advanced malignant melanoma were studied after intravenous administration of 131I-labeled Fab fragments specific for p97, an oncofetal glycoprotein of human melanoma. In all, 47 gamma camera imaging studies were performed for the purpose of localization of metastatic deposits. In addition to tumor, 131I-Fab uptake was also seen in liver and kidney. 20 of these studies included simultaneous administration of both an 131I-labeled Fab specific for p97, and an 125I-labeled Fab not specific for p97. Blood clearance of p97-specific Fab was significantly more rapid than for nonspecific Fab. Eight of these patients had biopsies of subcutaneous nodules at 48 and 72 h postinjection in order to assess whether localization of radioactivity was antigen specific. Antigen-specific localization was observed with average ratios of specific/nonspecific uptake of 3.7 (48 h) and 3.4 (72 h); uptake was strongly correlated with tumor p97 concentration (r = 0.81, P less than 0.01). Also, imaging studies of the bio-distribution of 131I-labeled anti-p97 Fab in patients selected for high p97 tumor concentration showed avid tumor uptake and more prolonged retention of labeled Fab in tumor than in normal tissues. Based on these studies, we estimated that total 131I doses of 500 mCi could be safely given to patients before dose-limiting toxicity would be observed. Accordingly, in seven selected patients, phase I radiotherapeutic trials were begun. For improved radiation safety, we developed automated methods to label Fab fragments with up to 200 mCi of 131I. So far, a total of 12 individual therapeutic doses, ranging from 34 to 197 mCi of 131I-labeled to 5 to 10 mg of Fab, have been administered with excellent tumor localization and without major target organ toxicity. Cumulative doses ranged from 132 to 529 mCi 131I. Side effects attributable to the radiation were mild, with a transient drop slightly greater than 50% in platelet and absolute neutrophil counts being observed in the two patients who received cumulative doses greater than 500 mCi. In the combined series of 47 diagnostic and 12 therapeutic studies, four acute reactions were observed: one episode each of transient chills and fever; flushing and hypotension; and two skin rashes. All of these reactions responded promptly to symptomatic therapy. After multiple administrations of 131I-(anti-p97) Fab (IgG1), isotype-specific immunity was observed in three patients. In two of these patients it was possible to successfully reinfuse after immunity had developed with 131I-(anti-p97) Fab of a different isotype (IgG2a). Dosimetry estimates were performed based on the biodistribution of (131)I-Fab in these patients,and for every 100 mCi of (131)I-Fab given, tumor receives 1,040 rads; liver. 325 rads; and bone marrow, 30 rads. Marrow would be expected to be the critical organ, if doses >500 mCi (131)I-Fab are given. These studies demonstrated that, with proper precautions, large doses (of an (131)I-labeled murine Fab fragments immunologically specific for a human melanoma-associated antigen) could be safely given to humans by using repetitive intravenous injections.