The use of fungicidal treatments to protect cotton cellulose from damage by micro-orgainisms is likely to continue. Many noncellulosic materials, including some inorganic and metallo-organic compounds, seem to be capable of photosensitizing the degradation of cotton cellulose. Since the interpretation of much of the published work on this effect lacks precision because it relates to a system which has undergone progressive loss of noncelulosic compound during experiments involing outdoor weathering, work has been carried out on cotton fabrics treated with four typical copper fungicides and irradiated with light from a carbon arc under conditions such that there was no loss of added compound. Copper naphtenate, copper Cunimene, and copper carbonate all show evidence of causing enhanced breakdown of the fabric, especially at low concentrations. Higer concentrations afforded some measure of protection. Copper oxinate gave protection at all concentrations.