A rapid and reproducible method is described for the simultaneous purification of mononuclear and polymorphonuclear cells, based on the use of a discontinuous gradient of Percoll. Centrifugation at 600 X gav for 20 min resulted in separation of the mononuclear and polymorphonuclear cells into 2 distinct bands at the interfaces. The upper layer contained the mononuclear cells and the lower band highly purified polymorphonuclear cells. Both populations were purified about 97% without detectable cell alteration.