This document specifies a method, using hypochlorite, to determine the mass percentage of protein fibre, after removal of non-fibrous matter, in textiles made of mixtures of certain non-protein fibres and certain protein fibres, as follows: wool, other animal-hair (such as cashmere, mohair), silk, protein, with cotton, cupro, viscose, modal, acrylic, chlorofibres, polyamide, polyester, polypropylene, glass, elastane, elastomultiester, elastolefin, melamine and polypropylene/polyamide bicomponent.