Our earlier reports (1-3) have provided a general, qualitative description of X-ray sensitivity of Chinese hamster cells as a function of their age with generation cycle. With a partially synchronized cell population and colony s used as an assay, it was established that cells were least sensitive to X-rays latter part of the DNA synthetic period (S period) and appeared to be about sensitive in the pre-DNA synthetic period (G1) and post-DNA synthetic pe (G2). The variations in sensitivity throughout the cycle seemed mainly bu exclusively due to variations in the threshold or shoulder of the survival curve. We were, however, aware of the faCt that even a few resistant S cells present ing stages we had designated as Gi or G2 could significantly alter the surviva for Gi and G2 cells. Such S cells may be present not only because of inadequa the synchronization procedure but also because of differences in the rate of pro sion of cells through the cycle which tends to desynchronize the population if the number of S cells present were known, their exact contribution to the su curve would be difficult to estimate, especially if the sensitivity of S cells throughout the S period. To determine X-ray sensitivity more accurately stages, we have used lethal amounts of tritiated thymidine (H3TdR) to ina the S cells. Furthermore, since the previous technique could not be used to the sensitivity of mitotic (M) cells, we have modified our procedure so that this be accomplished. Finally, in view of the possible association between X-ray sistance and DNA synthesis, it was important to find out whether cells in S constant or variable sensitivity throughout the synthetic period.