Both tetracycline (TET) and demethylchlortetracycline (DMC) are photosensitive substances under the conditions of this experiment. Ultraviolet wavelengths which pass through common window glass (>315 mμ) will activate this response in human skin. DMC appears to have greater photoresponse potential per unit concentration than tetracycline. A hot quartz lamp appears to be an inadequate light source for these studies, when compared with natural sunlight.