The photobleaching of undyed wool is shown to occur selectively in the spectral region of the causative light. For example, exposure to blue-violet (436 mμ) light causes higher reflectance (and transmittance), mainly near this wavelength. Similar wavelength correspondence or correlation was observed also at 405 and 546 mμ in the visible and at 365 and 389 mμ in the uv. Thus, wool tends to conform spectrally to the light to which it is exposed, by becoming permanently bluer, in blue light, greener in green light, whiter in white light, etc. Red light also bleaches wool but the photons are too weak to produce a well-defined maximum. The interpretation of this is that a part of the wool macromolecular structure, by absorbing radiant energy of a given wavelength, satisfies certain energy requirement and no longer absorbs, but reflects and transmits subsequent photons of that wavelength.