Inhibition of growth of LS174T human colon cancer xenografts in athymic nude mice due to 131I-labeled MoAb 17-IA treatment was compared to inhibition due to different single doses of 60Co external radiation. From those data, conditions which produced equivalent radiobiological end points could be identified and compared to dose estimates calculated using a technique analagous to the Medical Internal Radiation Dose (MIRD) Committee formalism. The tumor growth rate in mice injected with a single intraperitoneal administration of 300 μCi of 131I-labeled MoAb was reduced relative to tumor growth in untreated control animals and in mice administered unlabeled MoAb and was found to be similar to the growth rate of tumors given a single 6 Gy dose of 60Co radiation. Furthermore, the growth rate of tumors in mice that received three injections of 300 μCi of 131I-labeled MoAb on days 9, 16 and 28 after tumor cell injection was similar to the growth rate of tumors given a single 60Co dose of 8 or 10 Gy. The biodistribution data for 125I-labeled 17-1A MoAb were used to calculate total doses for the tumor and various normal tissues in animals given a single administration of 131I-labeled 17-1A MoAb. The absorbed radiation dose in tumor was approximately five times higher than in normal tissues. The results of the present study indicate that the tumor growth inhibition produced by the administration of radiolabeled antibody can equal that produced by up to 10 Gy of external beam radiation. In addition, the MIRD calculations allow comparison of this form of low dose radiation to external photon irradiation. © 1990.