When 12 purified protein preparations, including oval-bumin, lactalbumin, hemocyanin, gliadin, edestin, and other vegetable proteins, were exposed for 15-min. periods in shallow dishes to the radiation of a quartz mercury vapor arc, a characteristic odor and yellow color were produced, the temp. at which the proteins coagulated when heated was lowered. These changes were independent of the presence of water or air in the environment. Edestin, gliadin, and diluted egg albumin showed increased levorotation. Under certain conditions, the pH, not only of the protein solutions themselves, but of distilled water, was decidedly increased by the irradiation.