Ficoll is used in isolating lymphocytes from whole blood in areas such as histocompatability testing, in vitro cell-mediated immunity assays, and other procedures requiring pure lymphocyte preparations. Since this method can be adapted to very small volumes of blood, it is particularly suitable for isolation of lymphocytes where only limited quantities of blood are available, such as from cadavers and small children. Because of the rapidity and simplicity of the method, it is also the method of choice in emergency tissue typing procedures.