Abstract—The kinetics of the flavin‐sensitized photoinactivation of trypsin were investigated. Essentially similar results were obtained with riboflavin and with riboflavin‐5′‐phosphate (FMN) as sensitizers, while flavin‐adenine dinucleotide had only a very small sensitizing effect. The rate of the inactivation reaction was found to be directly proportional to light intensity over the range examined. The time‐course of the trypsin inactivation as measured by three assays could be fitted to first‐order kinetics.The rate of trypsin inactivation was identical as measured by the casein and benzoylargi‐nine ethyl ester assays, while a more rapid rate was observed as measured by the hemoglobin assay. These results are interpreted by postulating the existence of a “damaged” class of molecules after illumination uhich appears to be labile to post‐illumination heating or to 5Murea.Thet37value for trypsin inactivation, as corrected for the fraction of light absorbed, increased linearly with dye concentration. The trypsin concentration dependence of the reaction was found to be different in acid and basic solution. Trypsin had an inhibitory effect on the rate of photobleaching of the dye. Our results do not conform fully with any one of the several mechanisms that have been proposed to expl