CD14, a 55kDa glycoprotein, serves as a lipopolysaccharide (LPS) recognition molecule. CD14 is a monocyte differentiation antigen expressed by myeloid-derived cells, or other cells such as hepatocytes, as either a membrane-bound protein or a soluble serum protein. Increasing evidence indicates that soluble CD14 in plasma is an acute-phase protein derived, among other sources, from liver cells. Although information is available on the cellular expression of CD14, little is known about the cis- and trans-acting factors that regulate basal CD14 transcription in liver cells. We show here that liver cells have a relatively high basal CD14 transcription rate as determined by nuclear run-on assay. We cloned and sequenced an 883bp 5'-flanking region of the rat CD14 gene and demonstrated functional promoter activity in liver cells. Sequence analysis revealed that, like in the human and mouse CD14 genes, multiple Sp1 and AP1 binding elements exist in rat CD14. Site-directed mutagenesis and transient transfection assays demonstrated that an Sp1 element located at -836 and an AP1 element located at -270 are required for basal promoter activity in liver cells. Electrophoretic mobility shift assays indicate that both Sp1 and Sp3 nuclear factors interact with the -836 Sp1 element, while the AP1-related proteins Fra-2 and JunD bind to the AP1 motif. These data provide novel insights into the regulation of basal CD14 expression in liver cells.