Protein electrophoresis in polyacrylamide gel in the presence of sodium dodecyl sulfate is a widely used technique for the analysis of proteins. This article presents research on the binding of model azo-dyes, based on nongenotoxic benzidine analogs, with wool-fiber keratin. Protein fractionation, performed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and depending on the protein weight, was conducted on whole wool fibers, on fibers descaled in formic acid, and on lyophilized formic acid extracts. Dyes were bound with the proteins from the high-sulfur-protein area or high-tyrosine-protein area, depending on the level of dye dissolution. Fluorescence microscopy and transmission electron microscopy were used to illustrate the extent of the penetration of the model azodyes into the wool fibers. Scanning electron microscopy was used to detect changes in the fiber topography resulting from the chemical treatments. (C) 2003 Wiley Periodicals, Inc.