Abstract: A substance secreted or shed by muscle cells and white blood cells is disclosed. This novel substance is active in inhibiting proliferation of tumor cells and proliferation of stimulated lymphocytes. The substance does not inhibit proliferation of normal cells. This substance is used in accordance with the invention for treatment or prevention of cancer and the level of said substance in a body fluid or in a fluid conditioned by the growth therein of cells withdrawn from the individual, is used for the diagnosis of cancer or the level of risk of the individual of developing cancer.