Chinese hamster V79 cells were exposed to ionizing radiations of a wide range of linear-energy transfer (LET), including 145kV x-rays and six different heavy ions accelerated in the Berkeley heavy-ion linear accelerator. The LET of the ions ranged from 19 keV/um to 2000 keV/um. Survival curves were determined for both synchronized and asynchronous cells, using survival of colony-forming capacity as the end-point. Results with asynchronous cultures were similar to results reported previously for mammalian cells. There was increased effectiveness of killing per dose with increased LET until a change in shape of the single-cell survival curves resulted, from sigmoidal to exponential, with carbon ions (LET of 190 keV/um). With heavier ions, exponential curves were obtained, but with decreased effectiveness per unit dose. Synchronized cultures were obtained by mitotic selection. The expected Chinese hamster cell-cycle survival curve variation was found for X-rays, mainly reflecting the variation in the single-cell extrapolation number, with late S-phase cells the most resistant to radiation. When synchronized cultures were irradiated with the heavy ions that produce exponential survival curves; the survival curves were independent of the cell-cycle time of irradiation. With radiations of LET values between the low and high extremes, a reduced cell-cycle survival curve variation was found, indicating a gradual reduction in the cell-cycle survival curve variation as a function of increased LET.