In order to estimate the effect of soluble keratin protein (SKP) as a finishing agent for textiles, the influence of the additive treatment with the keratin on the dyeabilities of wool, cotton, cellulose acetate and polyester fabrics were investigated. In using the acid dye, K/S value in Kubelka-Munk equation and the rate of dye uptake of cotton fabrics were significantly increased by the treatment. From the amounts of SKP released from the fabrics through dyeing and washing processes, it was suggested that the increase of K/S value of cotton fabrics with high add-on is due to the well-dyed protein on the fabrics. In the case of the cotton fabrics with low add-on, SKP on the fabrics may promote adsorption and dispersion of the dye on the fiber surface, which result in acceleration of dispersion into the fibers at the initial stage of dyeing.