Conditioned medium from cultures of phytohemagglutinin-stimulated leukocytes has been assessed for its content of factors able to stimulate either colony formation by human granulopoietic progenitor cells, or 3H-thymidine incorporation by peripheral leukocytes from leukemic patients. The cell concentration, concentration of phytohemagglutinin and time required for optimal production of factors were investigated. Production of both classes of factors was found to be insensitive to inhibitors of cell proliferation and DNA synthesis, but sensitive to inhibition of protein synthesis by cycloheximide. The production of colony stimulating activity showed a markedly greater sensitivity to cycloheximide than did production of thymidine-incorporating activity. For cycloheximide and most of the other inhibitors studied, factor production by leukemic cell populations was affected to a somewhat greater extent than production by normal leukocytes. Separation by velocity sedimentation of the cells responsible for production provided no evidence for distinct classes of cells producing these two classes of factors.