A method for quantitatively measuring white blood cell (leukocyte) count and/or white blood cell (leukocyte) subsets count involves adding specific antibodies labeled with a marker to the biological fluid sample, capture of white blood cells from a fluid sample by a retainer, removal of other than leukocyte cells and other interfering substances by washing or using specific magnetic beads, and reading the result. The device for use in the present method includes a retainer for leukocyte cells and an absorption pad for taking up all excess washing solution flowing past the sample.