We have generated a humanized mouse model expressing GARP in place of the murine counterpart. mRNA and protein expression of human GARP indicates successful generation of the humanized model. Humanization of GARP does not change the overall development, differentiation or distribution of immune cell types, CBC profile, or liver chemistry in mice. Anti-GARP antibody treatment reduced tumor growth in B-hGARP mice in combination with anti-mouse PD-1, indicating that B-hGARP mice provide a powerful model for in vivo evaluation of novel combination therapies.