The feasibility of covalent enzyme immobilization into a bovine serum albumin-poly(ethylene glycol) (PEG) hydrogel was demonstrated with acid phosphatase (AP). The apparent kinetic parameters of the immobilized enzyme were determined using a plug flow reactor. No shift of the optimum pH was observed. The apparent K-m increased by two to five times immobilization, according to the molecular mass of PEG involved in the hydrogel’s structure. The internal diffusional restrictions decreased when the size of the microbeads was decreased and also when the chain length of PEG was increased, as the result of corresponding increase in the hydrogels porosity. The operational stability of the AP at 37 degrees C was increased markedly after immobilization. According to the molecular mass of PEG used, the half-lives ranged from 85 to 200 h as compared to a half-life of 72 min for the native enzyme tested under the same reaction conditions.