Granulocyte-colony stimulating factor (G-CSF) and granulocyte macrophage-colony stimulating factor (GM-CSF) are synthetic (man-made) versions of substances naturally produced in your body. These substances, called colony stimulating factors, help the bone marrow to make new white blood cells. When certain cancer medicines fight your cancer cells, they also affect those white blood cells that fight infection. To help decrease the risk of infections when these cancer medicines are used, colony stimulating factors may be given. Colony stimulating factors are also used to help the bone marrow recover after bone marrow transplantation and stem cell transplantation. They are also used to increase the stem cell count in the blood so that adequate number of stem cells can be collected for purposes of transplantation.