Colony-stimulating factor, which specifically stimulates mouse bone marrow cells to proliferate in vitro and generate colonies of granulocytes, or macrophages, or both, was purified 3500-fold from mouse lung-conditioned medium. Analysis by discontinuous polyacrylamide gel electrophoresis in the presence and absence of sodium dodecyl sulfate indicated that there was a single protein component. All of the colony-stimulating activity was coincident with the protein band. The molecular weight of colony-stimulating factor estimated by gel filtration was approximately 29,000 and by electrophoresis approximately 23,000. The specific activity of purified colony-stimulating factor from mouse lung-conditioned medium bound to concanavalin A-Sapharose, indicating that it is a glycoprotein. The small percentage of colony-stimulating factor in mouse lung-conditioned medium which did not bind to concanavalin A-Sepharose appeared to represent molecules which lacked the carbohydrate moieties required for binding to this lectin. It was necessary to include low concentrations (less than 0.01%, v/v) of polymers such as gelatin and polyethylene glycol, or nonionic detergents such as Triton X-100, in all of the buffers used throughout the purification scheme, otherwise colony-stimulating factor was lost from solution. At high concentrations (greater than 20 mug/ml) the factor stimulated the formation of granulocytic, macrophage, and mixed colonies from C57BL mouse bone marrow cells. As the concentration of purified colony-stimulating factor was decreased, the frequency of colonies containing granulocytes also decreased. At low concentrations of colony-stimulating factor (less than 70 pg/ml) only macrophage colonies were stimulated.