A microdosimetric analysis of previously published data on 211At-albumin, free 211At, and 211At-C215 irradiation of Colo-205 cells in a slowly rotating single-cell suspension is presented. A custom-built computer program based on the Monte Carlo method was used to simulate the irradiation and the energy deposition in individual cell nuclei. Separate simulations were made for the assumption that the 211Po atom stays in the position where it is created, and that it diffuses away. The mean event number at which 37% of all cells survived, n37, and the frequency mean specific energy per event, zF, were estimated. The Poisson distribution of events and simulated single and multievent distributions of specific energy were used to find the single-cell specific energy at which the probability of survival is reduced to 37%, z37. The calculated single-cell radiosensitivity values show that 211Po atoms, created on a cell surface by the decay of 211At atoms, will diffuse from the cell during its life-span. The increasing distance to the cell nucleus will drastically decrease the probability of the emitted alpha particle to hit the nucleus. This will result in fewer alpha-particle events in the cell nucleus. For dispersed cells, the diffusion of 211Po atoms will reduce the total dose from cell-bound 211At by a factor of 2.