Radioimmunolocalization of an 111In-labeled, mouse antimelanoma monoclonal antibody (MAb), ZME-018, was examined in 30 patients with metastatic malignant melanoma. Each patient received a single iv infusion of MAb at concentrations ranging from 0.6 to 40 mg, coupled to 5 mCi 111In by the chelating agent pentetic acid. No toxicity was observed in any patient. Total-body and region of interest scans performed at 4, 24, and 72 hours following MAb administration revealed uptake in 110 of 171 previously diagnosed metastases for a sensitivity of 64%. Nonspecific uptake of radioactivity was consistently observed in the liver and spleen, and less frequently in the bowel, testes, axillae, and bone. Sensitivity of detection increased significantly at doses of MAb above 2.5 mg, with 74% of the lesions imaging at 20 mg/5 mCi compared with 29% at 2.5 mg/5 mCi (P less than .005). Sensitivity actually decreased slightly at the 40-mg dose. There was a significant correlation between tumor uptake of MAb-111In-conjugate and increasing tumor size. Soft tissue lesions, such as skin and lymph node metastases, were imaged to a greater extent (77%) than were visceral metastases (40%). Mean plasma clearance of ZME-018 was prolonged with a half-life of 33.6 hours in patients receiving 40 mg, compared with 17.8 hours in patients given 2.5 mg (P less than .01). Urinary excretion of the isotope averaged 11.4% of the injected dose over 48 hours. Hence radioimmunolocalization of melanoma with 111In-ZME-018 appeared feasible. The sensitivity of the technique varies with MAb dose, specific activity of 111In-MAb conjugate, tumor size, and disease site.