Abstract: A method is disclosed for differentiating white blood cells. The method uses at least one surfactant and at least one dilute acid which together selectively strip the cytoplasm from certain classes of white blood cells and not others. More particularly, the method causes lysis of lymphocytes, monocytes, eosinophils and neutrophils, but not basophils. Thus, basophils are differentiated from other PMN sub-classes by their appearance as intact cells. The method totally avoids the need for dye preparation and the vagaries of staining techniques and can be used either manually or with instrumentation.