Imbalance in cytokine homeostasis is thought to play an important part in the pathogenesis of chronic inflammatory diseases such as rheumatoid arthritis. We demonstrated that T cells might exert a pathological effect through direct cellular contact with monocyte-macrophages, inducing a massive upregulation of IL-1β and tumour necrosis factor (TNF) 1. We showed that this mechanism that might be relevant to chronic inflammation is specifically inhibited by high-density lipoproteins (HDL) 2. Like many other stimuli, besides proinflammatory cytokines, the contact-mediated activation of monocytes induces the production of cytokine inhibitors such as IL-1Ra. HDL inhibited the production of IL-1β and TNF but not that of IL-1Ra induced in monocytes activated by membranes isolated from stimulated T cells to mimic cellular contact. This was also the case in peripheral blood mononuclear cells stimulated by either phytoheamagglutinin or tetanus toxoid. Similarly, IL-1Ra mRNA expression was not inhibited contrary to IL-1β and TNF mRNA. This demonstrates that different molecules at the surface of stimulated HUT-78 cells are involved in the induction of IL-1β, TNF and IL-1Ra in monocytes, IL-1β and TNF being activated by HDL-specific ligand(s). Separation of CHAPS-solubilized membrane molecules by liquid isoelectric focusing showed that two activity peaks were present; one activating IL-1β, TNF and IL-1Ra production, the other inducing the production of IL-1Ra in the absence of IL-1β and TNF. Further isolation of these two types of factor by gel filtration demonstrated that factor(s) inducing IL-1β, TNF and IL-1Ra displayed a Mr around 40,000 kDa, whereas factors inducing IL-1Ra only displayed Mr around 70,000 kDa and 30,000 kDa. Thus different factors are expressed at the surface of stimulated T cells that differentially trigger the production of proinflammatory and anti-inflammatory factors, and are differently affected by HDL.