Colony stimulating factor 1 (CSF-1) is a functionally versatile, circulating homodimeric growth factor that stimulates the survival, proliferation, and differentiation of mononuclear phagocytic cells, the differentiation of osteoclast progenitor cells and that regulates cells of the female reproductive tract. CSF-1 is also expressed in the central nervous system where it may regulate the differentiation and activation of microglia. The diverse forms of CSF-1 are all encoded by a single gene. Alternative posttranscriptional splicing and posttranslational cleavage determines whether CSF-1 will be produced as a secreted proteoglycan, secreted glycoprotein, or as a cell-surface glycoprotein that may be involved in cell-cell interactions. CSF-1 is expressed in glioblastoma cell-lines, normal human astrocytes, and in operative specimens of human glioma. The CSF-1 receptor, encoded by the c-fms proto-oncogene, is also expressed in human gliomas. We conclude that coexpression of CSF-1 and its receptor in some human gliomas hints at a possible autocrine or paracrine growth stimulatory role for CSF-1; however, its function in the mammalian CNS remains to be elucidated.