Disclosed are a crosslinked gelatin microspheres containing CSF and a water soluble CSF-gelatin conjugate. Both the microspheres and the water soluble conjugate provide an improved CSF stability. They have a high potentiation on the antitumor activity of macrophages in respect of the CSF amount and the time required for macrophage activation and are effective in maintaining their activated state for a long period, compared with the native CSF. The mechanism of macrophage activation by the microspheres containing CSF is mediated via phagocytosis and different from that by native CSF, which is believed to activate macrophages via cell surface receptor. The species specificity of CSF may be abrogated when the CSF is internalized into macrophages through phagocytosis.