The expression of CD14, a monocyte receptor for the bacterial lipopolysaccharide (LPS), is upregulated during monocytic cell differentiation. Although a Sp1 site at -110bp of the CD14 promoter was shown to be critical for activation of the promoter during the differentiation, how the Sp1 site is regulated has not been well understood. We have recently reported that expression of MEF2D protein increases during the differentiation of HL60 promyeloid cells to monocyte and that the upregulation of the protein is required for CD14 expression during the differentiation [Mol. Immunol. 36 (1999) 1209]. However, there is no obvious MEF2 binding site in the critical region of the CD14 promoter. In this study, which aimed to determine the regulatory role of MEF2D in monocytic cell differentiation, MEF2D was found to form a complex with Sp1 in U937 promyeloid cells. Transient transfection experiments showed that co-expression of MEF2D and Sp1 synergistically activated the CD14 promoter. The results support a model in which increased MEF2D protein during monocytic cell differentiation activates the CD14 promoter through interaction with Sp1.