Color evaluation, surface morphology, amino acid composition, solubility and molecular weight distributions are used to describe the intitial degradative changes in Bombyx mori silk fibroin resulting primarily from irradiation with UV and visible light. The biochemical techniques are more sensitive detectors of the initial chemical alterations of the fibroin molecule than either color evaluation or morphology. The data suggest selective destruction of amino acids accompanied by peptide bond cleavage. The peptide bond cleavage mechanism is initially selective and becomes random with increased irradiation.