[en] Gene conversion of two trp5 alleles has been determined in a derivative of strain D.7 of Saccharomyces cerevisiae exposed to 150 kVp X-rays or to ³H β-rays (by suspension in tritiated water). Precautions were taken to ensure that pre- and post-irradiation conditions were similar for X- and β-irradiated sub-sets of the same population. At similar dose rates, stationary phase cells showed a linear dose response to both radiations. When different dose rates of ³H β-rays were delivered, there was no indication of increased induction per unit dose at lower dose rates. The observed yield per unit dose after ³H β-irradiation was slightly less than that obtained with 150 kV/sub p/ X-rays. Further experiments to determine the RBEs in this system are in progress