Hydrophilic organic colloids such as collagen or gelatin are modified for use in photographic elements such as film or paper, or for use as reagents in automated dry chemical analyzers. The modification comprises reaction of some of the carboxy groups attached to the polypeptide with (i) a amide bond forming agent, e.g. 1-pyrrolidinylcarbonylpyridinium chloride, and (ii) a di- or triamine, such as piperazine, diethylenetriamine or ethylenediamine. Such modification enables that colloid to react faster with a gelatin hardener such as bis(vinylsulfonyl)methane (BVSM). When coated over an equal amount of unmodified gelatin, and both layers imbibed with BVSM, a modified gelatin layer showed an enzyme resistance greater than that of the unmodified gelatin. This demonstrates that the modified gelatin hardened preferentially. This discovery of a method for differential hardening can be utilized to prepare multilayer photographic elements or multilayer reagents for chemical analyzers that contain one or more layers which are hardened preferentially, i.e. to a greater degree than other layers.