An object of the present invention is to provide a separation system and a separation material for efficiently separating white blood cells or mononuclear cells from a biological fluid containing blood cell components. White blood cells or mononuclear cells can be efficiently separated by contacting a biological fluid containing blood cell components with a separation material having an average fiber diameter of at least 2.0 µm but not more than 6.0 µm and an air permeability coefficient M of at least 6.2 but not more than 35 to capture white blood cells on the separation material, and recovering the captured white blood cells or mononuclear cells using a detachment solution.
