Monoclonal antibodies Ost6 and Ost7 (mouse IgG1) to human osteogenic sarcoma were isolated from ascitic fluid and labeled with radioiodine. After injection into athymic nu/nu mice with s.c. xenografts of human osteogenic sarcoma, the uptake of radioactivity in tumors, visceral organs and blood was determined. Five days after injection, Ost6 and Ost7 showed preferential accumulation in tumors (tumor:blood ratio, 4.3). With testicular and bladder tumors, both unreactive with Ost7, there was no localization of radiolabeled Ost7 in xenograft growths. When Ost7 was labeled with 131I, its accumulation into human osteogenic sarcoma could be clearly visualized by whole-body .gamma.-scintigraphy without computer-assisted data processing.