The wavelength dependence for 8-methoxypsoralen (8-MOP)-sensitized inhibition of scheduled DNA synthesis was investigated in the epidermis of albino hairless mice. Topical (0.1%) applications of 8-MOP followed by exposure to narrow bands from a monochromator in the range of 300-380 nm produced a dose-dependent inhibition of DNA synthesis. Prior treatment with 8-MOP did not alter the dose-dependent inhibition of DNA synthesis following exposure to 300 nm and to 310 nm. By contrast, DNA synthesis inhibition following exposure to UVA wavelengths was seen only after treatment with 8-MOP. An action spectrum, constructed from the dose-response regression lines, showed peak effectiveness at 335 nm. Since the therapeutic usefulness of psoralen photochemotherapy may be related to inhibition of cell proliferation, it is suggested that light sources with peak emission in the 335-nm region would be more efficient than the commonly employed UVA blacklights.