Radioimmunotherapy (RIT) employing radiolabeled monoclonal antibodies (MAb) against tumors has been extensively tested for more than two decades. Although results of RIT in patients with malignant lymphoma are encouraging, the results in other types of tumors are less favorable in bulky disease. In contrast, several reports suggest the role of RIT as an adjuvant treatment of cancer. However, in treating small lesions, especially when cancer cells are spread in the peritoneal cavity but do not produce tumors, loss of decay energy outside of small lesions occurs because of millimeter path length of β particle. In contrast, previous works have indicated that RIT employing β emitters effectively treated lesions smaller than their path length. In this article, our experimental results in animal models of colon cancer cells are summarized to support feasibility of adjuvant RIT in minimal disease.