A dye diffusion method using fluorescence microscopy for monitoring the penetration of solvents into keratin fibers was investigated. The lipid stain, Nile red, was used as a probe. Differences in the penetration of aliphatic alcohols into dry wool, as revealed by the extent of dye penetration, correlated well with the known variation in sorption rate for solvent molecules of varying molecular size and shape. The penetration of various animal fibers by t-butanol was examined. T-butanol readily penetrated the cortex of human hair, whereas the cortices of wool, mohair, and cashmere fibers showed no penetration under the same conditions. However, t-butanol appeared to penetrate the intercellular material between the cuticle cells of these fibers.