IN a previous communication1 a new isolation technique was described by which pure prekeratin can be obtained from the epidermis of the cow's nose. For the extraction of the epidermis 0.1 M citric acid–sodium citrate buffer of pH 2.6 and ionic strength of 0.6 was used and prekeratin was isolated from the epidermal extract by serial precipitations in the pH range of 4.5–7.0. The prekeratin obtained by this method has a molecular weight of 640,000 and consists of highly asymmetric particles which reveal a typical α-keratin pattern in X-ray diffraction investigation. The chemical composition of this prekeratin was recently determined and the results are reported here.