Abstract: To find a method for determining the immunoreativity of monoclonal antiboies after radiolabeling avidin is unlabeled and labeled with Rodamine, 131I and 188Re, respectively. The affinities and halfdisorbed amounts of biotin and four kinds of avidin are determined by the biotin columns plus nonlabeled avidin(cold avidin). The affinities of biotin and avidin unlabeled and labeled with Rodamine, 188Re and 131I are decreased in turn. Their halfdisorbed amounts from biotin are 21.9,19.5,25.7 and 47.9 μg of cold avidin. Two kinds of radiolabeled avidin have lower affinity with biotin than that of avidin unlabeled and labeled with Rodamine. There is a possibility to evaluate the reativity of biological materials with different labeling methods by avidinbiotin system.