Interleukin-4 (IL-4) is the prototypic type 2 immunoregulatory cytokine that can suppress the production of many monocyte and macrophage pro- inflammatory mediators. In this study we investigated the regulation by IL-4 of IL-12 and IL-10 production. While IL-4 suppressed lipopolysaccharide (LPS)-induced IL-12 and IL-10 production by human peripheral blood monocytes, IL-4 suppressed LPS-induced IL-12, but not IL-10, production by synovial fluid mononuclear cells from patients with rheumatoid arthritis. IL-4 also suppressed IL-12, but not IL-10 production, by LPS-stimulated in vitro monocyte-derived macrophages. Similarly, IL-4 cannot suppress LPS-induced tumour necrosis factor-α (TNF-α) production by synovial fluid cells and monocyte-derived macrophages. The failure of IL-4 to regulate IL-10 production is not due to the failure of IL-4 to suppress TNF-α, and vice versa. The data suggest that the IL-4 receptor subunit, γ(c), is essential for IL-4 regulation of LPS-induced IL-10 production and that a correlation exists between duration of monocyte culture, reduction in γ(c) mRNA in cultured cells and hyporesponsiveness of monocyte-derived macrophages to IL- 4 for regulation of LPS-induced IL-10 production. This study highlights the importance of investigating responses to IL-4, as a potential therapeutic anti-inflammatory agent, by cells isolated from inflammatory sites and not by the more easily accessible blood monocytes. This study emphasizes the involvement of signalling from γ(c) in IL-4 regulation of LPS-induced IL-10 production by monocytes and macrophages.