A reagent, satisfying the undermentioned conditions, is useful for counting the number of leukocytes and measuring the hemoglobin concentration in blood samples. The reagent is free of cyanides and it is able to maintain hemoglobin in a stable state for a prolonged time. The reagent contains at least one cationic surfactant selected from the group consisting of a quaternary ammonium salt of a concentration capable of hemolyzing erythrocytes in the blood and oxidizing hemoglobin and a composition of pyridinium salts; at least one cationic, nonionic or amphoteric surfactant; and at least one hemoglobin stabilizer.