[en] Dose-response relationships for unstable chromosome exchange aberrations were obtained after irradiation with 200 kV X-rays and ⁶⁰Co gamma rays, the doses ranging within 0.05-3.0 Gy. The data points were fitted to the linear quadratic model Y = C + αD + βD², and after the chromosome hits leading to two-break unstable aberrations were estimated, to the model average x = C +kD. The results fitted the latter model particularly well, the index of determination being 0.988 for gamma rays and 0.997 for X-rays. The RBE of 200 kV X-rays as compared with ⁶⁰Co gamma radiation was 1.6, when primary chromosome breaks leading to dicentric and centric ring aberrations were used as the biological endpoint. (author)