IL-6 is a potent regulator of T-cell activation, proliferation and differentiation. Since IL-10 inhibits cytokine production by T cells, the effect of IL-10 on IL-6 production by T cells was investigated. IL-6 production by purified monocytes or T cells was detected from cell-free culture supernatants by ELISA after stimulation of the cells with LPS or an anti-CD3 monoclonal antibody for 3 days. Although the main source of IL-6 are LPS activated monocytes (29.6 +/- 10 ng/ml), T cells secreted sufficiently high levels of IL-6 (790 +/- 200 pg/ml) to stimulate the high affinity IL-6 receptor. IL-10 decreased anti-CD3 induced IL-6 mRNA expression by up to 80%. In addition, IL-10 significantly inhibited IL-6 release from T-cells. Highly purified, anti-CD3 activated T-cells secreted 600 +/- 150 pg/ml IL-6 compared to 21 +/- 2 pg/ml IL-6 following addition of IL-10 (10 ng/ml; P < 0.001). FACS analysis revealed a monocyte contamination of the T-cell preparations of less than 0.5%. In addition, no IL-1 production was detectable. Thus, in our experiments the effect of IL-10 on IL-6 production was independent of the presence of monocytes. Finally, inhibition of IL-6 production was not reversed by IL-2 (100 U/ml). In conclusion, IL-10 suppressed the synthesis of IL-6 by T-cells via a monocyte- and IL-2-independent mechanism. These results may help to understand the complex regulation of T-cell mediated cytokine production by IL-10.