Appropriate validation of any bioassay to be used in the characterization of biological products is critical. In this study, we report a validation study of a flow cytometry-based assay to measure the complement-dependent cytotoxicity (CDC) of a biosimilar candidate monoclonal antibody (Mab) directed to CD20 antigen, as indicative of its biological activity. The assay was validated by examining: assay robustness, specificity, repeatability and intermediate precision. It demonstrated to be robust for all factors evaluated. It also showed a high level of specificity and was found to be free of interference through the validation process. The degree of precision (Cvs < 7%) obtained in this study was satisfactory. The presented work demonstrated that a flow cytometry-based cytotoxicity assay is a suitable method in the lot to lot quality control monitoring of the culture supernatant and an active pharmaceutical ingredient of 1B8 Mab.