A new type of hydrogel capable of sol-gel phase-reversible transition by changes in the environmental glucose concentration has been synthesized. The hydrogel consists of vinylpyrrolidinone-allylglucose (VP/AG) copolymer and concanavalin A (Con A). The formation of the hydrogel is based on the specific interaction between glucose on the copolymer and glucose receptor sites on Con A. Hydrogels were formed immediately after mixing the copolymer and Con A. The critical factors in the formation of hydrogel were the concentrations of the copolymer, of glucose on the copolymer and of Con A. In general, the formation of a hydrogel became more efficient as the concentration of glucose on the copolymer decreased and/or as the Con A concentration increased. The hydrogel formed became a sol in the presence of glucose in the environment. The environmental glucose concentration necessary to dissolve the gel to sol was approximately four times higher than the concentration of glucose on the copolymer. Upon removal of the environmental glucose by dialysis, the sol became a gel again and the sol-gel phase transition could be repeated.