We describe a method for in vitro isolation of mononuclear cells from peripheral blood or bone marrow using a Fenwal CS3000 Apheresis device without employing density gradients or sedimenting agents. The automatic processing program requires minimal operator intervention and no subjective operator decisions. A mean of 67% of starting mononuclear cells were recovered in a 100 ml product having 95% mononuclear cells and less than 1% of the original red blood cells. The average processing time was 35 minutes.