Human lymphocytes from a cell culture were irradiated with laser pulses of an Xe-Cl excimer laser (308 nm) or a dye laser at wavelengths from 312 to 640 nm. After conversion of photoinduced DNA damage into DNA strand breaks the "comet assay" (single-cell gel electrophoresis) was used to determine the number of photons required to induce detectable DNA damage. Between 308 and 450 nm the number of photons necessary for DNA damage increases about 10,000 times from 2.14 x 10(8) to 2.85 x 10(12) photons. Between 308 and 318 nm the number of photons necessary to induce detectable DNA damage is proportional to e0.45 lambda. Between 340 and 450 nm it is proportional to e0.05 lambda. No detectable damage was induced at 540 and 640 nm after irradiation with 9900 kJ m-2. Two-photon effects are unlikely to play a significant role.