ESR studies have been done on natural and UV-irradiated silk fibroins and wool keratins at the temperature range of -196degreesC to 20degreesC. The intensities of ESR signals obtained from the irradiated samples at -196degreesC remarkably increase with respect to those of natural samples. While the signals mainly consist of triplet peaks at -196degreesC, a doublet arises around the room temperatures. Foe the first time, at room temperature without any external effect the complicated ESR spectra of fibrous proteins (wool keratin and silk fibroin) whose components are as follows have been observed: (1) (for white wool keratin) a central doublet with DeltaH(m) = 1.1 mT and g = 2.0075 DeltaH(m) = 5 mT and g = 2.1911; (2) a wide peak with DeltaH(m) approximate to 66 mT and g approximate to 2.1575 (3) the 'sulfur' peak given in the literature with DeltaH(m) approximate to 2.2 mT and g = 2.0218 (4) the signal with DeltaH(m) = 0.6 mT and 2.0065, and for silk fibroin. (a) a very wide signal with DeltaH(m) approximate to 70 mT and g approximate to 2.084 (b) a very sharp signal with DeltaH(m) approximate to 1.1 mT and g approximate to 2.01; and (c) relatively narrower signal with DeltaH(m) approximate to 5 mT and g approximate to 2.336.