Following a report that nerve growth factor preparations have granulocyte‐colony‐stimulating activity, we investigated the presences of colony‐stimulating factors in 7s mouse submaxillary nerve growth factor and its subunits. Macrophage colonies were formed in mouse bone marrow cultures after exposure to preparations of 7s nerve growth factor, the γ subunit, and, to a small extent, the α subunit: the β subunit, which is responsible for the nerve growth function, did not stimulate colony growth. Furthermore, the esteropeptidase activity of the γ subunit was not detected in preparations of macrophage colony‐stimulating factor purified from the giant cell tumor (GCT) cell line. Immunoprecipitation of radiolabeled γ subunit witha polyclonal antibody to L‐cell macrophage colony‐stimulating factor showed a protein band that could represent the γ subunit of nerve growth factor. Separation of the marcophage activity from the esteropeptidase activity of the γ subunit was accomplished on the basis of molecular size. Thus, macrophage colony‐stimulating factor was a contaminant of nerve growth factor produced by the mouse submaxillary gland and copurified with the γ subunit.