This paper describes a radical new, one-step method for the routine isolation of the mononuclear cells from human blood in high yield and purity. In this procedure the blood sample is mixed with a Nycodenz separation medium prior to centrifugation. Erythrocytes and polymorphogranulocytes are pelleted during centrifugation leaving the mononuclear cells in the supernatant. The method can be used for a wide range of blood sample volumes and can be carried out with greater ease than the currently accepted methods.