A simple method that allows affinity measurements of antibodies to integral membrane proteins is described. Kinetic Exclusion Assay was used to determine the concentration of free antibody that remains in solution after equilibrium has been established between the antibody and the cell-surface-expressed antigen, from which the equilibrium dissociation constant (Kd) was determined. It eliminates the requirement for soluble antigen and modifications such as radio-labeling or fluorescent labeling of the antibody. For one of the cell-surface-expressed antigens, it was determined that the affinity of the antibody to the cell-surface-expressed antigen was similar to that of the purified, soluble form of the antigen. In addition to the simplicity of the approach, the method provides a true measure of the affinity/avidity of the antibody to the native form of cell-surface-expressed targets, including antigens that cannot be produced in soluble forms, and to unknown cell surface antigens.