The promise of antibody therapeutics has been greatly expanded by the development of monoclonal antibody technology and more recently antibody humanization. By transferring the mouse antibody binding site into a human antibody gene, we can engineer a "human antibody" which retains the specificity and biological effects of the original mouse antibody but has the potential to be nonimmunogenic in humans. Additionally, antibody effector functions can be improved through manipulation of the antibody constant region genes. We have produced a humanized version of OKT3 with human IgG4 and kappa constant regions. This antibody retains all of the in vitro characteristics of murine OKT3 including induction of cytokine release and T cell activation markers. Humanized OKT3 has an affinity of 1.4 x 10(9) M-1 relative to a 1.2 x 10(9) M-1 affinity of murine OKT3. Substitution of a glutamic acid for leucine at residue 235 in the antibody constant region abrogates FcR I binding and causes a marked reduction of T cell activation. The humanized FcR mutant of OKT3 has potential to be an improved therapeutic for transplantation and may have applications in autoimmune disease treatment.