The receptor for interleukin-6 (IL-6) consists of two subunits: a ligand specific IL-6Ralpha and gp130 that is responsible for signal-transduction. A soluble form of the ligand specific chain was described that when complexed to IL-6 is capable of binding to the membrane-bound gp130 subunit and thus can elicit signal-transduction. This soluble receptor can act on cells that express only the gp130 but not the ligand-specific subunit of the IL-6R. This phenomenon, called trans-signaling, introduced a novel aspect of cytokine action. In this study we examined the response of Jurkat cells, that are known not to express IL-6Ralpha, to IL-6, the soluble IL-6 receptor (sIL-6R) and a covalent complex of IL-6 and sIL-6R termed Hyper-IL-6. We studied the expression of tumour necrosis factor (TNF) and interferon-gamma (IFN-gamma). The complex of IL-6+sIL-6R and Hyper-IL-6 inhibited significantly the production of TNF in a gp130-dependent manner, whereas no differences in IFN-gamma expression were found. IL-6 and sIL-6R alone were not effective. Because we did not detect major differences in the TNF mRNA levels upon treatments, we conclude that the inhibition of TNF production should occur at the post-transcriptional level. These results provide another example of trans-signaling and underline the physiological importance of sIL-6R, and in the case of Hyper-IL-6 its possible therapeutic application can also be considered.