T cell-derived IL-4 inhibits the expression of several inflammatory cytokine genes. In this study, IL-4 downregulated TNF-alpha and -beta, IL-1 beta IL-6 and IFN-gamma steady-state mRNA and protein production by blood leukocytes. In a detailed kinetic analysis, cytokine mRNA levels were significantly decreased in the presence of IL-4, suggesting an early and prolonged downregulatory effect. These findings support the general thesis that IL-4 is an important regulator of the inflammatory immune response.