The murine mAb 22 (M22) binds to the human high affinity receptor for the Fc portion of IgG (Fc gamma RI). This mAb recognizes an epitope on Fc gamma RI that is distinct from the natural ligand (Fc) binding site, and, therefore, can bind to Fc gamma RI in the presence of saturating levels of IgG found in vivo. Fc gamma RI is expressed only on cytotoxic effector cells and it acts as an efficient mediator (trigger molecule) of effector functions. Directing tumor cells or other pathogens to Fc gamma RI utilizing M22 conjugated to an anti-tumor- or to an anti-pathogen-specific mAb may represent an effective method to enhance their removal and destruction under physiologic conditions. Humanization of M22 may improve the therapeutic potential of the mAb since murine mAbs normally elicit a human anti-mouse Ab response. In this report we describe the humanization of M22 by CDR-grafting onto human V region frameworks based on KOL VH and REI V kappa attached to human IgG1 and kappa constant domains. This humanized mAb, H22, displays equivalent binding specificity and affinity as its murine counterpart. In addition, H22 is able to trigger superoxide release from Fc gamma RI-bearing cells, a finding that demonstrates that H22 triggers Fc gamma RI functions. The successful humanization of mAb 22 is an important step in the development of Fc gamma RI-directed immunotherapy.