A method for measuring the occupancy rate of the first specific binding substance in a cell population, in which the cell population is brought into contact with the second and third specific binding substances, and the second and third specific binding substances are contacted. The first, second, and third specific binding substances all bind to the target cell surface protein, and the occupancy rate is the target cell surface protein expression, including measuring the number of cells bound by. The proportion of cells to which the first specific binding substance is bound, the second specific binding substance competes with the first specific binding substance, and the third specific binding substance is Does not compete with the first specific binding substance, and the formula: (1- (number of cells bound to the second specific binding substance / number of cells bound to the third specific binding substance)) × A method in which the value calculated by 100 is the occupancy rate (%).細胞集団における第１の特異的結合物質の占有率を測定する方法であって、細胞集団に第２及び第３の特異的結合物質を接触させることと、第２及び第３の特異的結合物質が結合した細胞数を計測することと、を含み、第１、第２及び第３の特異的結合物質はいずれも対象細胞表面タンパク質に結合するものであり、占有率は、対象細胞表面タンパク質発現細胞のうち、第１の特異的結合物質が結合した細胞の割合であり、第２の特異的結合物質は、第１の特異的結合物質と競合するものであり、第３の特異的結合物質は、第１の特異的結合物質と競合しないものであり、式：（１－（第２の特異的結合物質が結合した細胞数／第３の特異的結合物質が結合した細胞数））×１００により計算される値が占有率（％）である、方法。
