The growth and differentiation of colonies of granulocytes and/or macrophages in semisolid cultures of hemopoietic cells requires the presence of "colony stimulating" factors (CS factors). Colony stimulating factors are found in serum, urine and certain tissues, and in media "conditioned" by certain cells in culture. A procedure has been developed for purification of CS factor from human urine. The 100,000-fold purified product is active on mouse bone marrow cells at less than or equal to 10(-11) M but is only partially purified. Polydispersity of activity has made further purification difficult and may necessitate use of biologically selective techniques such as immunosorbent chromatography. Knowledge of the properties of human urinary CS factor has been derived from the in vitro assay of biological activity following various treatments. It appears to be a sialic acid-containing glycoprotein of molecular weight 45,000-60,000 daltons that migrates electrophoretically between alpha1 globulin and albumin. Highly purified CS factor preparations can be iodinated with five atoms of iodine per molecule of protein without loss of biological activity. Neutralizing antibody to human urinary CS factor neutralizes CS factor from other human and monkey sources. Higher concentrations of antiserum are required for neutralization of murine CS factor. Human urinary CS factor has similar properties to human and murine CS factors that stimulate the early appearance of macrophages in murine colonies. It is physically distinct from CS factors that stimulate the formation of pure granulocytic colonies by murine or human hemopoietic cells. Purified CS factor of the human urinary type is being used to investigate the nature of the interaction of CS factor with target cells and its physiological role in vivo.