The concentration of committed granulocytic progenitor cells (CFU-C) in functionally unique subpopulations of human peripheral blood mononuclear cells was determined by the in vitro methylcellulose assay. Using immunoabsorbent column chromatography and rosette-depletion techniques, it was demonstrated that CFU-C, although not present in either purified T [thymus derived] or B [bone marrow derived] lymphocyte populations, are highly concentrated in the null cell population, which lacks sheep erythrocyte receptors and surface immunoglobulin. Further fractionation of this null subset demonstrated that CFU-C do not bear complement receptors, but require the presence of peripheral blood mononuclear cell feeder layers for maximum proliferation.