We developed appropriate conditions to use a laser with 60 femtosecond pulses, a frequency of 1 KHz and a wavelength of 266 nm to efficiently crosslink proteins to DNA in human nuclei for the purpose of using immunoprecipitation to study the binding of specific proteins to specific sequences of DNA under native conditions. Irradiation of nuclei for 30 min with 1-3 GW/cm(2)pulses crosslinked 10-12% of total protein to DNA. The efficiency of crosslinking was dose and protein specific. Histones H1 and H3 were crosslinked by 15 min of irradiation with 20-25% efficiency, at least 10 times more strongly than the other histones, consistent with experiments using conventional UV light. Irradiation for 15 min did not damage proteins, as assayed by SDS-PAGE of Ku-70 and histones. Although the same level of irradiation did not cause double-strand breaks, it did make the DNA partially insensitive to Eco RI restriction enzyme, probably through formation of thymidine dimers. Immuno-analysis of crosslinked nucleoprotein showed that Ku crosslinking to nuclear DNA is detectable only in the presence of breaks in the DNA, and that nucleosomes are bound to a significant fraction of the telomeric repeat (TTAGGG) (n).