Wild-type Chinese hamster cells CHO K1 and their radiosensitive mutant xrs5 were irradiated at 308 nm, using light pulses of a XeCl excimer laser with total energy fluences of 0.1 kJ/m2 to 4.08 kJ/m2. Chromosome-type and chromatid-type chromosome aberrations have been observed at pulse irradiances of 2.5 x 10(7) W/m2 and 1.7 x 10(8) W/m2, indicating that in mammalian cells DNA double-strand breaks occur already in this irradiance range. The results obtained with laser irradiation are compared with X-ray irradiation.