Oncostatin M (OSM), an IL-6 subfamily cytokine, inhibits proliferation and causes morphological changes in many tumor cell lines. GM-CSF, phorbol-12-myristate-13-acetate (PMA), and lipopolysaccharide (LPS) induce OSM expression. To investigate the mechanisms governing OSM promoter activity, we have cloned and partially sequenced an 8.5 kb fragment of human genomic DNA immediately 5' of the OSM coding region and mapped the transcription start site. Transient transfection assays with a series of 5' deletion plasmids demonstrated maximal reporter activity in U937 cells with a minimum 304 bp construct. The 5'-proximal region of the human OSM gene contains a C/EBP consensus element around -45 bp and several GC-rich regions around -60, each of which is responsible for basal promoter activity. Electrophoretic mobility shift assay coupled with supershift analysis confirmed the presence of a cis -acting binding site for activated STAT5 complexes following GM-CSF treatment. Furthermore, transient transfection studies demonstrated a loss of GM-CSF responsiveness in reporter constructs containing mutations within this STAT element. Our results establish that C/EBP and an as yet unidentified GC-rich binding transcription factor are responsible for basal OSM promoter activity, while GM-CSF-stimulated OSM expression is driven by activated STAT5 complexes binding to a cis -acting STAT element on the OSM promoter.