Injection of avidin can decrease the background radioactivity due to a radiolabeled biotinylated monoclonal antibody. We compared the chase effects of avidin, streptavidin, neutravidin, and avidin-conjugated ferritin on a radiolabeled antitumor monoclonal antibody in tumor-bearing nude mice. A radioiodine-labeled biotinylated monoclonal antibody (OST7) was administered to athymic mice bearing osteogenic sarcomas. After 24 h, an avidin, streptavidin, neutravidin or avidin-conjugated ferritin chaser was intravenously injected into the mice. At 2 h after the chase, the biodistribution of the radiolabeled monoclonal antibody was determined. Clearance from the blood was dose-dependently accelerated by avidin and its effect was 10-fold stronger than that of neutravidin or avidin-ferritin. Streptavidin did not promote clearance of the biotinylated antibody. Avidin was the most effective chasing agent for improving the biodistribution of the radiolabeled biotinylated monoclonal antibody among the four avidin derivatives tested.