Radioisotopes with moderate half-lives are essential for conventional radioimmunotherapy using tumor-selective MAbs which require days for localization. Actinium-225, with a half-life of 10 days and a yield of 4 alpha particles in its decay chain, may be an ideal choice for tumor-targeted radioimmunotherapy. Release of daughter radioisotopes from the primary chelator after the first decay has been a complication with the use of 225Ac. It has been reported that the domain-deleted product of MAb CC49, Hu-delta CH2 CC49, is able to extravasate and penetrate more deeply into tumors than the parent IgG molecule. We reasoned that once the 225Ac-chelate-MAb had penetrated into the tumor, the daughter radioisotopes would remain trapped even if they had been released from the primary chelator. Actinium-225 HEHA MAb CC49 conjugates were tested for distribution, micro-distribution and therapy in immunocompromised mice which had LS174T tumors growing at subcutaneous or intramuscular sites. Both 125I and 225Ac CC49 and Hu-delta CH2 CC49 were efficient in delivery of the radioisotopes to tumor sites. Tissue micro-autoradiography for the two antibody forms did not demonstrate any differences in micro-distribution of either 125I or 225Ac in the tumor. Furthermore, there was no detectable difference for the two carriers in the tumor retention of daughter radioisotopes from 225Ac. Therapy experiments with 225Ac were complicated by radiotoxicity of the conjugates. The lethal dose was about 0.5 microCi in two strains of mice regardless of the carrier. At injected doses of 0.5 and 0.25 microCi, CC49 was slightly active in tumor stasis, whereas no consistent significant effect of 225Ac-Hu-delta CH2 CC49 on growth of tumors was observed. The potential of 225Ac in radioimmunotherapy is limited by the radiotoxicity of its daughter radioisotopes. Its potential will only be realized if stable conjugates, capable of daughter radioisotope retention, can be devised.