We propose a novel polymeric hydrogel system as a cell container that can encapsulate cells in the polymer networks reversibly. A phospholipid polymer bearing 2-metha- cryloyloxyethyl phosphorylcholine units and p-vinylphenyl- boronic acid (PMBV) was synthesized to form a covalently cross-linking hydrogel with a polymer having multivalent hydroxyl groups such as poly (vinyl alcohol) (PVA). The hydrogel was formed not only in water but also even in the biological medium. However it was dissociated by addition of low molecular weight divalent hydroxyl compounds such as glucose. After fibroblast cells were added to PMBV solution and mixed with PVA, the cells were immobilized into the hydrogel. After dissociation of the hydrogel, the cells were adhered and proliferated as usual on a conventional cell culture plate. That is, the hydrogel system is useful to maintain the cells as a cell container.