An investigation of the efficacy of astatine-211—tellurium colloid for the treatment of experimental malignant ascites in mice reveals that this α-emitting radiocolloid can be curative without causing undue toxicity to normal tissue. By comparison, negatron-emitting phosphorus-32 as colloidal chromic phosphate had no antineoplastic activity. The most compelling explanation for this striking difference is the dense ionization and short range of action associated with α-emission. These results have important implications for the development and use of α-emitters as radiocolloid therapy for the treatment of human tumors.