Chinese hamster V79 cells in culture were exposed to astatine-211, an alpha-particle-emitting radiohalogen. The dose-log survival response was linear with no detectable shoulder. Cells in monolayers had a D0 of 1.0 microCi/ml. Suspended cells had a D0 of 0.60 microCi/ml with a cellular uptake of 2.5 fCi/cell; this is equal to approximately 1.5 alpha-particle traversals per cell nucleus. The frequencies of chromosome and chromatid breaks were linear with dose, but the number declined rapidly with time. These data are discussed in relation to published alpha-particle beam studies and the potential use of 211At in radionuclide therapy.