Idiopathic pulmonary fibrosis (IPF) is an insidious lung disease with no known cure or effective therapy. Macrophage-derived insulin-like growth factor-I (IGF-I) is thought to play a role in the pathogenesis of IPF, however, little is known about the control of IGF-I expression in macrophages. In this report we investigated the cisregulatory elements that control basal expression using luciferase reporter constructs in RAW264.7 macrophages. We show that the +95 to +329 region contains elements necessary to direct maximal promoter activity whereas the +251 to +329 region contains the minimal promoter. Mapping transcriptional start sites for endogenous IGF-I in primary macrophages revealed that the major transcriptional start site is centered at +150 whereas the most 3'transcriptional start site is centered at +255. Nuclear proteins from primary and RAW264.7 macrophages bind specifically to the region required for maximal promoter activity (+134 to +173) and to the region required for minimal promoter activity (+267 to +299). Antibody supershift assays indicate that Sp3 bound to the +267 to +299 region. Moreover, mutation of the putative binding site reduced Sp3 binding in EMSAs and increased promoter activity in luciferase reporter gene assays. We also found that the regions from -1711 to -855 and -855 to -337 contain putative macrophage specific suppressor elements that do not function in HeLa or Cos-7 epithelial cell lines. These data support the view that macrophage IGF-I expression is positively regulated by elements located in the 5'-untranslated region and negatively regulated by elements in the 5'-flanking region of the IGF-I gene.