Disclosed is a method for determining a white blood cell count of a whole blood sample, which comprises: mixing a whole blood sample with a surfactant to thereby obtain a mixture; allowing the mixture to stand for a time sufficient to lyze the white blood cells contained in the whole blood sample and release intrinsic myeloperoxidase from the white blood cells; measuring the concentration of the released myeloperoxidase in the mixture; and determining the white blood cell count in the whole blood sample, based on the concentration of the released myeloperoxidase. Also disclosed is a method of the present invention, in which, in addition to a white blood cell count of a whole blood sample, the concentration of C-reactive protein contained in the same whole blood sample is measured.