A method for measuring an occupancy rate of a first specific binding substance in a cell population, including bringing a second and a third specific binding substance into contact with the cell population and counting the number of cells to which the second and the third specific binding substances binds, in which all of the first, the second, and the third specific binding substances are specific binding substances that bind to a target cell surface protein, the occupancy rate is a proportion of cells to which the first specific binding substance binds, in cells expressing the target cell surface protein, the second specific binding substance competes with the first specific binding substance, the third specific binding substance does not compete with the first specific binding substance, and the occupancy rate (%) is a value calculated by an expression: (1 - (Number of cells to which the second specific binding substance binds/Number of cells to which the third specific binding substance binds)) × 100.
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