A one-step procedure for purification of mononuclear and polymorphonuclear cells from human blood is described. It is a modification of the Hypaque-Ficoll method with density of 1.095 g/ml. Centrifugation at 200 X g for 20--30 min resulted in the separation of mononuclear and polymorphonuclear cells into 2 distinct bands at the interface. The mononuclear cell fraction contained 83.9 +/- 1.6% lymphocytes and 13.8 +/- 2.3% monocytes, while the other conssisted of highly purified neutrophils (96.4 +/- 1.0%). Leukocyte recovery by this method was always greater than 80% and viability exceeded 98%. Both cell fractions retained their immunological functions.