The human Fc gammaR-binding inhibitor can be used to inhibit the non-specific Fc-gamma receptor (Fc gammaR)-mediated binding of mouse monoclonal antibodies used for flow cytometric analysis of human tissue. Four different classes of Fc gamma receptors have been identified: Fc gammaRI (CD64), Fc gammaRII (CD32), Fc gammaRIII (CD16), and Fc gammaRIV, and they are expressed at varying levels in multiple cell lineages including high expression in myeloid, granulocyte and B cells. The biological function of the Fc gammaR, including initiation of endocytosis, phagocytosis and antigen presentation, is elicited upon binding of host-immunoglobulin. The extent to which mouse monoclonal antibodies will bind to human Fc gammaR varies depending on the isotype of the monoclonal antibody. Furthermore, different monoclonal antibodies of the same isotype will display different binding to human Fc gammaRs.