Leuco-naphthazarin dyes white human hair fibers in an aqueous solution under the exceedingly mild conditions without any reducing agents and strong alkali in a dye bath at room temperature. The color development can be achieved without any oxidants. The dyeability remarkably increased by an addition of benzyl alcohol. The dye exists almost in cuticle cells in dyeing in an aqueous solution, while it penetrates easily in cortex cell in the presence of benzyl alcohol. Wash-fastness was excellent and the dye was not removed at all in an 25% aqueous pyridine solution under the refluxing conditions. The microphotomeric spectra and Raman spectra of crosssectional dyed fibers showed that leuco-naphthazarin was not simply oxidized to afford naphthazarin in the fibers. It is strongly suggested that chemical interaction exists between the dye and the fibers. Contrary to leuconaphthazarin, the sorption of leuco-quinizarin did not take place at all even in the presence of benzyl alcohol.