A novel protein adsorbent consisting essentially of a porous copolymer obtained by copolymerizing a monomer mixture comprising at least one cyano group-containing monomer and at least one cross-linkable monomer, said copolymer having an average pore diameter (d) of from 40 A to 9,000 A and a total pore volume of from 0.05.sqroot.X ml to 1.5.sqroot.X ml per gram of the copolymer in a dry state, in which X designates the weight proportion of said cross-linkable monomer expressed in terms of the percent by weight based on the total monomers. The present porous copolymer adsorbent has a remarkably high protein adsorbing capacity. The adorbent can advantageously be used for various purposes such as purification and separation of proteins, and removal of proteins. Copolymer-protein composites obtained by adsorption of proteins on the present porous copolymer also are industrially useful and have a wide variety of applications.