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Radiation-induced cancer of bone, liver, and lung has been a prominent harmful side-effect of medical applications of alpha emitters. In recent years, however, the potential use of antibodies labeled with alpha emitting radionuclides against cancer has seemed promising because alpha particles are highly effective in cell killing. High close rates at high LET, effectiveness under hypoxic conditions, and minimal expectancy of repair are additional advantages of alpha emitters over antibodies labeled with beta emitting radionuclides for cancer therapy. Cyclotron-produced astatine-211 (21aAt) and natural bismuth-212 (2_ZBi) have been proposed and are under extensive study in the United States and Europe. Radium-223 (a23Ra) also has favorable properties as a potential alpha emitting label, including a short-lived daughter chain with four alpha emissions. The radiation dosimetry or" internal alpha emitters is complex due to nonuniformly distributed sources, short particle tracks, and high relative specific ionization. The variations in dose at the cellular level may be extreme. Alpha-particle radiation dosimetry, therefore, must involve analvsis of statistical ener_, deposition probabilities for cellular level targets. It must also account full), for nonuniform distributions of sources in tissues, sourcetarget geometries, and particle-track physics. INTRODUCTION Alpha particles are energetic, positively charged helium nuclei emitted from the nuclei of heavy, elements because the binding energy