Alpha-Chymotrypsin was light sensitized by acylating with cis-cinnamoyl ester, a substrate interconvertible to the trans form by ultraviolet (UV) light. The degree of acylation by this method was complete leaving no residual activity of the enzyme. Upon UV irradiation the inhibited enzyme regained about 70% of its original activity, thereby adding light-sensitiveness to the proteolytic enzyme. In seeking a photographic application of the light-sensitized enzyme, a pigmenting enzyme was incorporated with it. The coupled enzyme system was shown to exhibit a light signal in the form of dark pigment slurry.