Lymphocyte Separation Medium (LSM) is a sterile-filtered iso-osmotic solution designed for the in vitro isolation of lymphocytes from diluted whole blood.

The use of LSM is based on the method of isolating lymphocytes using density gradient centrifugation (differential centrifugation), resulting in the formation of several distinct cell layers. Mononuclear cells (lymphocytes and monocytes) and platelets will be contained in the banded plasma-LSM interphase due to their density. Erythrocytes and granulocytes will migrate through the gradient to the bottom of the tube. Lymphocytes can be recovered by aspirating the plasma layer and removing the cells.