The ability to perform biological studies on Natural Killer (NK) cells requires effective methods for their isolation from hematopoietic cells of other lineages. NK cells are a discrete lymphocyte subset distinguishable from B- and T-lymphocytes on the basis of both physical and phenotypic characteristics that can be exploited for then purification. Techniques based on differential cell buoyancy (centrifugation on discontinuous density gradients, such as Percoll [1]) have been used to enrich NK cells from mixed lymphocyte populations, but do not allow purification of these cells to homogeneity. The mononuclear cell suspensions obtained, although enriched in NK cells, also contain variable proportions of other cell types (notably monocytes and/or activated T- and B-lymphocytes) (2) and subsets of NK cells of higher density are lost in these preparations.