Human peripheral lymphocytes were irradiated with different doses of 3-MeV electrons. The dosimetry was performed according to Fricke. The frequency of different aberration types was determined and the dose-response relation was calculated. The data were fitted by least-squares regression analysis to different models. The dicentric, dicentric + centric ring, and different acentric data gave the best fit to the linear quadratic model. The RBE of 3-MeV electrons compared with 220-kV x-rays increases with increasing dose.