Reactive partially reduced oxygen species such as superoxide anion (O2-), hydrogen peroxide (H2O2) and hydroxyl radical (OH) are produced in aerobically growing organisms during normal cellular respiration. To provide an effective defense against these reactive species, many aerobic organisms have evolved a multienzyme defense which includes superoxide dismutase, catalase and peroxidase. The superoxide anion may cause appreciable cellular damage by oxidizing aminoacids or by causing DNA strand breakage. Catalase was covalently immobilized on activated methoxypolyethyleneglycol-5000 and catalase and PEG-catalase were encapsulated in erythrocyte. Enzyme activity, encapsulation yield and hemograme analysis were determined for each sample. The erythrocyte shape of the samples were investigated by using phase contrast microscopy.