The invention is directed to a monoclonal antibody directed against CD20 antigen, for therapeutic administration to humans, wherein each of the light chains of the antibody is encoded by murine-human chimeric nucleic acid sequence SEQ ID No. 27, and each of the heavy chains of the antibody is encoded by murine-human chimeric nucleic acid sequence SEQ ID No. 19. The invention is further directed to methods of in vitro activation of FcγRIIIA receptors in immune effector cells with the antibody and methods of treating CD20-expressing leukaemia or lymphoma with the antibody.