Abstract: Disclosed is a method for separating leukocytes from platelets. Blood serum is added to a centrifuge tube. A suspension containing leukocytes and platelets is then added to form a layer on top of the blood serum, the layer having a density less than the density of the blood serum. The cells and the layers are centrifuged at a force and for a time sufficient to separate the leukocytes from the platelets. The resulting, isolated leukocytes are then recovered from the centrifuge tube. In some embodiments, a suspension of the leukocyte cells are pretreated by incubating with a stannous ion reducing agent, the suspension having a stannous ion concentration of from about 50 .mu.g to about 1000 .mu.g stannous ion per 10.sup.8 leukocyte cells. A suspension of the pretreated leukocyte cells is then incubated with a solution containing a technetium-99m (VII) salt, preferably sodium .sup.99m Tc-pertechnetate, to radiolabel the leukocyte cells.