Differences in clinical results reported with the use of iodine-125 encapsulated implants compared to those with gold-198, radon-222, and iridium-192 prompted a study of the RBE, relative to60 Co γ radiation, of X rays simulating the spectral distribution of radiation from encapsulated125 I. The parameter used for the RBE determination was the killing of surface-attached log-phase HeLa-S3 cells. The RBE was determined at dose rates varying from 15 to 130 rad/min which are about 90 to 500 times greater than those employed in therapy with interstitial ${}^{125}{\rm I}\ \text{implants}$ . It was found to be approximately 1.4 when measured as a ratio of $D_{0}\text{'}{\rm s}$ and 1.3 as determined on the basis of the ratio of doses for 10% survival (dose rates of 15 and 30 rad/min). The RBE values were found to be dose-rate dependent, increasing linearly from 130 to 30 rad/min and possibly leveling off below this point.