Reversible photoregulation of the activities of various proteins was achieved by two general approaches. One approach involves the covalent attachment of photoisomerizable components such as azobenzene units or thiophenefulgide components. In one of the photoisomer states of the photoactive units, the tertiary structure of the proteins is retained and, consequently, their biological functionalities are switched ＇＇ON＇＇. In the complementary photoisomer state, the structure of the proteins is distorted and its activities are switched ＇＇OFF＇＇. The hydrolytic activities of papain have been photostimulated by covalent linkage of azobenzene units. While trans-azobenzene-modified papain reveals bioactivity towards hydrolysis of N-alpha-benzoyl-D,L-arginine-nitroanilide (7), cisazobenzene-modified papain is switched ＇＇OFF＇＇ towards the hydrolytic process. Similarly, association of the saccharide 4-nitrophenyl alpha-D-mannopyranoside (11) to thiophenefulgide-modified concanavalin A is photostimulated. The second approach for controlling the activities of biomaterials involves their immobilization in photoisomerizable polymers. In these systems photostimulated transport of the protein substrates across the polymer matrices triggered ＇＇ON-OFF＇＇ the functions of the encapsulated biomaterial. alpha-Chymotrypsin was immobilized in the series of photochromic copolymers consisting of acrylamide and 1-beta-(methacryloxy)ethyl-3,3＇-dimethyl-6-nitrospiroindoline-2,2＇-[2H-1]benzopyran(l2), bis(4-dimethylaminophenyl)-(4-vinylphenyl)methyl leucohydroxide (14) or 4-(methacryloylamino) azobenzene (15). The hydrolysis of N-(3 -carboxypropionyl) -L-phenylalanine p-nitroanilide (13) by alpha-chymotrypsin is controlled by the permeability properties of 12 across the different photoisomer states of the respective polymers.