A previous analysis of non-stochastic dose (Jostes et al., Radiat. Res. 127, 211-219, 1991; Schwartz et al., Health Phys. 62, 458-461, 1992) based on data obtained during irradiations of C-18 cells in suspension by alpha particles emitted from two forms (chelated and ionic) of 212Bi was made using survival curves. No appreciable difference in slope (1/D0) was found between the two forms. Such non-stochastic analyses do not account for the large differences in specific energies deposited in the individual cell nuclei. This microdosimetric (stochastic) analysis aims to determine the survival sensitivity (1/z0) of the individual C-18 cells using the distribution of specific energies deposited in the individual cell nuclei. The resulting sensitivity is greater for the alpha particles emitted from the chelated 212Bi than from the ionic 212Bi. An attempt to account for this greater sensitivity in terms of greater LET of alpha particles passing through the cell nuclei from the chelated 212Bi is unsuccessful. Instead the greater sensitivity disappears if the microdosimetric analysis uses average values for the radii of the cell and of its nucleus rather than the values (from the peak in the cell size distribution) used by the non-stochastic dose analysis.