We used a pretargeting technique to treat a nonobese diabetic/severe combined immunodeficient murine model of human adult T-cell leukemia with an anti-Tac antibody-streptavidin (HAT-SA) conjugate, which recognizes CD25, followed by bismuth 213 ((213)Bi)-1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA)- biotin. In the 3-step pretargeting radioimmunotherapy protocol, HAT-SA (140 or 400 microg) was administered intravenously (i.v.) to bind to the interleukin 2 receptor alpha (IL-2R alpha; CD25)-expressing tumor cells. After 24 hours, 100 microg of a synthetic clearing agent was administered i.v. to remove unbound circulating HAT-SA conjugate from the circulation. Four hours later, (213)Bi-DOTA-biotin was administered i.v. for therapy. Tumor growth was significantly inhibited in 3 trials by using 250 microCi (9.25 MBq) of (213)Bi-DOTA-biotin with a pretargeting technique as monitored by serum levels of soluble IL-2R alpha and/or human beta-2-microglobulin (P <.05, t test) and by survival of tumor-bearing mice in the treatment groups (P <.02, log rank test) as compared with the control groups. No prolongation of survival was observed with a nonspecific antibody-SA conjugate or in the absence of the radionuclide. Additionally, no prolongation of survival resulted from administration of (213)Bi directly linked to intact HAT. Furthermore, there was no prolongation of survival when the beta-emitting radionuclide yttrium 90 instead of the alpha-emitting radionuclide (213)Bi was used. The pretargeting approach with (213)Bi inhibited tumor growth more effectively than did immunotherapy with unmodified HAT. The best results were obtained with combination therapy that involved (213)Bi-DOTA-biotin with a pretargeting technique supplemented by 4 weekly doses of HAT. The findings of this study support the use of this combination approach in a clinical trial in patients with IL-2R alpha-expressing leukemias.