Peripheral blood is the primary source of lymphoid cells for investigations of the human immune system. Its use is facilitated by the Ficoll-Hypaque density gradient centrifugation method described here. It is a simple and rapid method of purifying peripheral blood mononuclear cells (PBMC) that takes advantage of the density differences between mononuclear cells and other elements found in the blood sample. The mononuclear cell sample can be purified from monocytes by adherence or by exposure to L-leucine methyl ester; methods are described for both procedures. Cord blood and peripheral blood from infants contain immature cells, including nucleated red cells, that can result in significant contamination of the mononuclear cell layer, and removal of these cells requires additional steps that are described. The isolation procedures presented here can also be applied to cell populations derived from tissues.