Experiments in vitro, using V-79 Chinese hamster fibroblasts, were carried out in order to establish whether or not there was a difference between the relative biological efficiency (RBE) and/or the oxygen enhancement ratio (OER) of 50 and 4 MV bremsstrahlung photons. Dosimetry was performed with both Fricke dosimetry and ionization chamber. Cells were irradiated in all glass-metal system under both oxic and anoxic conditions and subsequently plated for cloning assay. Dose corrections were made for the backscatter from the glass to which the cells were attached during irradiation. A statistically significant difference between RBE of the two energies was found. The RBE was estimated to be 1.1 for 50 MV photons. No difference in OER was found. This RBE difference is of clinical interest as it is generally accepted that it should be possible to determine the dose level with a precision of about +/- 5% in radiation treatment.