Interleukin-12 p70 (IL-12p70) is a key cytokine produced by dendritic cells (DC) able to drive the development of T helper type 1 (Th1) lymphocytes. We showed that thymic and other fibroblasts strongly inhibit IL-12p70 production by splenic DC stimulated by lipopolysaccharide plus either anti-CD40 or interferon-gamma (IFN-gamma) and by purified splenic DC stimulated by Pansorbin plus IFN-gamma. This IL-12p70 inhibitory activity is secreted in the conditioned medium of primary fibroblasts and fibroblast cell lines but not by haematopoietic cell lines. As IL-10 was the unique factor able to inhibit IL-12p70 produced by cultured splenic DC, we showed that a neutralizing antibody to IL-10 did not suppress the IL-12p70 inhibitory activity of thymic fibroblast-conditioned medium (FCM). This FCM potently inhibits the maturation and expression of major histocompatibility complex class II and co-stimulatory molecules induced by stimulation of spleen-derived DC. While thymic FCM suppressed the IL-12p70 expression by stimulated spleen-derived DC, tumour necrosis factor-alpha production is not affected. This inhibitory activity is able to down-regulate the IL-12p35 subunit transcription and expression, resulting in the impaired assembly of IL-12p70 heterodimer. As fibroblasts are present in the tissue microenvironment and are active players in the establishment of an immune response, the nature and role of the fibroblastic inhibitory activity remain to be established.