In vitro studies of the relative biological effectiveness (RBE) of 50-MV X rays have shown an RBE of 1.1 relative to 4-MV X rays. This will be important in clinical radiotherapy. The aim of this study was to verify these results and to investigate whether photonuclear processes might cause the difference in RBE. To do so, 50- and 20-MV X rays and 50-MeV electrons were investigated with respect to RBE. Chinese hamster V79 cells were irradiated in a specially designed system which allows for a high reproducibility of geometry and dosimetry. Fractionation experiments were also carried out to establish the RBE at the clinically relevant dose level, 2 Gy. Fricke dosimetry was used, and the results were confirmed with ionization chamber measurements. The RBE for 50-MV X rays was estimated to be 1.14 at a surviving fraction of 0.1 and 1.12 at a surviving fraction of 0.01. The RBEs for the other qualities were equal to one. The RBE calculated for the 2 Gy/fraction experiments was 1.17.