A method is provided for separating a suspension of cellular material comprising at least two differently-sized cell types using a spinning membrane separator (10). The method comprises selecting the cell type to be separated by passing through the membrane (62)； determining a concentration of the selected cell type in the suspension； selecting an inlet flow rate for the suspension； selecting a rotational speed for the spinning membrane separator (10) related to one or more of the concentration and relative size of the selected cell type in the suspension； rotating the spinning membrane separator (10) at the selected rotational speed so that the selected cell type tends to migrate to regions of the shear field adjacent the porous membrane (62)； and flowing the suspension through the spinning membrane separator (10).
