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As it is an important challenge for pharmacists to access the stability of monoclonal antibodies because of the widespread of centralized preparation units, we conducted a study to evaluate the physicochemical and biological stability of diluted rituximab at 1 mg/mL over six months at 4 °C. We also conducted the study at 40 °C to demonstrate that all methods employed were stability indicating. Various protein characterization methods were used to determine changes in physicochemical properties of rituximab, including size-exclusion chromatography, dynamic light scattering, turbidimetry, cation-exchange chromatography, second-derivative ultraviolet and infrared spectroscopy, and peptide mapping. Cell culture was used to assess biological stability. We demonstrated that diluted rituximab stored at 4 °C in polyolefine bags remained stable for at least six months. No physical or chemical instability was observed, and the biological activity was fully maintained. Size exclusion chromatography did not show polymerization or fragmentation. No difference was noticed in the hydrodynamic diameter of RTX. No additional peak or decrease in the areas under curve was found by cation exchange chromatography. The thermal aggregation curves and their derived thermodynamic parameters were unchanged. The primary, secondary and tertiary structures of the protein were not modified. Finally, the direct cytotoxic effect of rituximab was fully maintained.