A polyoxazoline hydrogel having a disulfide bridge was prepared by the reduction-oxidation of thiol-protected poly(N-acetylethylenimine) (PAEI) (anthracene-SS-modified PAEI) or by the cross-linking reaction of the partially hydrolyzed PAEI with bifunctional disulfide compounds. The reduction of thiol-protected PAEI produced the thiol-modified PAEI, which was susceptible to air oxidation to form the networks. These gels basec on highly hydrophilic PAEI were stable and swollen in water as a hydrogel. A series of PAEIs having varying amounts of the functional groups were prepared and subjected to cross-linking reactions by reducyion-oxidation. The degree of swelling in water increased with decreasing the degree of substitution in the prepolymer