We have previously reported that glucocorticoids markedly increase and anti-glucocorticoids (such as RU-486) block c-fms RNA and protein expression in some breast cancer cell lines, but not in others, and that this increase is the consequence of increased transcription from the first, epithelial cell-specific promoter of the c-fms gene (encoding CSF-1R, macrophage colony-stimulating factor receptor). Employing DNaseI protection and electrophoretic mobility shift assays (EMSA), we now demonstrate that DNA-transcription factor protein complexes are formed on the c-fms first promoter at a composite regulatory element containing overlapping binding sites for AP-1 proteins, bHLH factors, and the glucocorticoid receptor (GR). Competition studies indicate that transcription factor proteins bind the AP-1 site and the GR element (GRE) and both GR and AP-1 proteins are involved in DNA-protein complex formation. The complexes differ in quantity and glucocorticoid inducibility in the different breast cancer cell lines studied depending on whether the promoter responds to glucocorticoid stimulation. Transient transfection of promoter/reporter gene constructs resulted in reduced basal transcription activity of this promoter and lack of glucocorticoid stimulation when the AP-1 site was mutated. We conclude that AP-1 proteins, GR and associated co-factors regulate transcription from the c-fms first promoter and that differences in recruitment of the various components are responsible for cell specific repression and activation of this gene in breast carcinoma cell lines.