Interleukin-10 (IL-10) expression was significantly elevated upon stimulation with lipopolysaccharide (LPS) when the sialic acid-recognizing Ig-superfamily lectin Siglec-5 or -9 was overexpressed in RAW264 cells. During the course to clarify the mechanism for this activation, we found that IL-10 promoter proximal region up to -500 bp led to transactivation similar to that up to -1,500 bp. Among the transcription factors that activate the mouse IL-10 promoter so far reported, the level of C/EBPβ was increased in Siglec-9-expressing cells. Transient expression of the C/EBPβ major isoform LAP led to an increase in the expression of IL-10 in Siglec-9-expressing cells, but not in mock-transfected control RAW264 cells upon stimulation with LPS, as assessed by either a luciferase assay or the production of IL-10. Without LPS, the IL-10 promoter was activated by transiently expressed LAP in Siglec-9-expressing cells, however, the magnitude of transactivation was less than that with the LPS stimulation. The knockdown of C/EBPβ down-regulated the production of IL-10. Taken together, these results suggest that one of the reasons for the stimulation of IL-10 expression in Siglec-9-expressing cells may be an increase in intracellular C/EBPβ level.