A polymerized product of protein, comprising a polymer of one or more protein-containing monomers represented by the general formula P.sub.r --�(Y).sub.k --(X).sub.m --R.sup.1 !.sub.n (1) wherein R.sup.1 represents a polymerizable group capable of causing a radical polymerization, P.sub.r denotes a protein residue, Y denotes a group formed from a functional group in said protein and the functional group R.sub.2 of the modifying agent given below, X denotes an organic divalent radical, k is zero or 1, m is zero or 1 and n is a numeral of 1 or higher, said protein-containing monomer being obtained by reacting a protein with a modifying agent represented by the general formula, R.sup.2 --(X).sub.m --R.sup.1 (2) wherein R.sub.2 represents a functional group capable of combining with a functional group of protein and R.sup.1, X and m are as given above.