To investigate possible undesirable effects due to the intravenous administration of a reagent of a xenogenic nature (monoclonal antibody 225-28S) in man, a toxicologic study was carried out on 85 patients with metastatic cutaneous melanoma. Two reagents were tested in this study: purified monoclonal antibody (MoAb) 225-28S and its F(ab')2 fragment. Purified MoAb was labelled with 131I and F(ab')2 fragment with 131I, or 123I, or 111In or 99Tc. The quantity of MoAb or F(ab')2 injected ranged from 14 to 750 micrograms, and the specific activity from 37.0 to 2116.4 MBq/mg. The total radioactivity injected varied from 25.9 to 891.7 MBq/mg. In addition to a careful clinical examination, the following tests were done to monitor possible adverse effects: blood glucose, azotemia, RBC, WBC, platelet count, serum creatinine, creatinine clearance, plasma electrolyte levels, serum proteins, albumin/globulin ratio, serum bilirubin, SGOT, SGPT, gamma GT, and CPK. These tests were done before the injection and on days 7 and 14. No patient experienced adverse general effects like fever, nausea, vomiting or allergic reactions. None of the aforementioned hematometric and biochemical tests showed significant variations compared with the initial values. It is concluded that a single injection of these reagents at the dosages tested is completely atoxic.