To obtain a crosslinking agent safe for various organisms of proteins and having effects on formation of an irreversible crosslinking agent, a sustained release agent or a DNA conjugate.

This protein conjugate-forming agent comprises a polyphenol as an active ingredient and is effectively used as a gene conjugate, a cell adhesion inhibitor or an immunological tolerance agent. The principal ingredient of the polyphenol is catechins composed of epigallocatechin gallate, tannin or protocyanidin. The catechins of the polyphenol are useful as a drug delivery formulation of cytokines which are physiologically active proteins by using the catechins of the polyphenol as the conjugate-forming agent for the proteins. Since the catechins of the polyphenol have excellent cell adhesion inhibitory actions, the catechins can be used within a wide range of prophylaxis or treatment such as an immunosuppressive agent, an antiallergic agent, an antimetastasis inhibitor, an eye drop, an ophthalmic perfusion, an antiinflammatory agent, a prophylactic or a therapeutic agent for arteriosclerosis, etc. Furthermore, the catechins can widely be applied even to cord blood, various species of cells and organs for transplantation.