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Cells synchronized by mitotic selection were irradiated at various times during the first generation cycle with 1.75 and 3.5 Gy of 31-MeV protons and then analyzed both for lethality and for chromosome aberrations. The results showed a maximum of radiosensitivity in late G/sub 1/-early S followed by an increase in resistance to a maximum in S in both endpoints of damage. For both types of effect, early to mid G/sub 1/ is definitely less sensitive than mid to late G/sub 1/. The ratios between the maximum and minimum sensitivity were approximately 1.4 and 2 for 1.75 and 3.5 Gy, respectively. These values agree with data reported to the literature on different mammalian cell lines exposed to X rays at comparable survival levels. For both dose values, the logarithm of the surviving reaction at various times of the generation cycle is linearly related to the corresponding chromosomal aberration frequency, with 37% survival corresponding to about one aberration per cell.