We investigated the wavelength dependence of cyclobutane thymine dimer and (6-4)photoproduct induction by monochromatic UV in the region extending from 150 to 365 nm, using an enzyme-linked immunosorbent assay with two monoclonal antibodies. Calf thymus DNA solution was irradiated with 254-365 nm monochromatic UV from a spectrograph, or with 220-300 nm monochromatic UV from synchrotron radiation. Thymine dimers and (6-4)photoproducts were fluence-dependently induced by every UV below 220 nm extending to 150 nm under dry condition. We detected the efficient formation of both types of damage in the shorter UV region, as well as at 260 nm, which had been believed to be the most efficient wavelength for the formation of UV lesions. The action spectra for the induction of thymine dimers and (6-4)photoproducts were similar from 180 to 300 nm, whereas the action spectrum values for thymine dimer induction were about 9- and 1.4-fold or more higher than the values for (6-4)photoproduct induction below 160 nm and above 313 nm, respectively.