The probability that a cell will survive a single passage of a high-LET ion is of interest in both radiation protection and radiotherapy. Below we derive a simple relationship which allows such a probability to be obtained. For monolayers of cells irradiated by high-LET ions with a single LET (track segments experiments), Charlton and Sephton (1) have shown that the probability of a cell surviving a dose of D to its nucleus can be written as exp (-D/)D0): )o0 the specific lethal dose, is given by -d/ln(1 d/Do). d is the dose to the nucleus per ion passage and Do is the average dose to the nuclei which reduces survival to 37%. 0o is a function of ion, cell type, and LET. d/Do is 1/no when no is the average number of passages at the Do dose. This treatment fits exponential survival curves for high-LET radiation.