Abstract α-Emitting radionuclides such as 211At have a number of physical characteristics which make them attractive for the treatment of micrometastases. 211At was bound to polymer microspheres and its efficacy was compared with the β-emitting 32P and 90Y colloids for the treatment of intraperitoneally growing K13 hybridoma tumors in mice. Single graded doses of 0.1–2.5 MBq 211At microspheres injected intraperitoneally 24 hr after inoculation of the hybridoma cells improved survival and produced higher cure rates than 32P colloid, 90Y colloid, or no treatment. One of the most striking contrasts between 211At microspheres and 90Y or 32P colloids was the ability of relatively low doses 211At to affect cures. When comparing the groups with the highest survival rate for each radionuclide (0.1–1 MBq 211At, 2.5 MBq 90Y, and 2.5 MBq 32P), 211At treatment resulted in an improved survival over that with 32P therapy, but the difference was not significant between 211At and 90Y. Toxicity studies with 211At microspheres showed that dosages up to 17 MBq per mouse were not lethal. In conclusion, the present study suggests that the high-energy transfer and the short-range cytotoxicity of the α-emitter 211At might be of benefit for intracavitary radiotherapy.