Résumé : The present study has shown a decreased production of IL-6 by LPS-stimulated mononuclear cells of B-CLL blood. This decrease with attributed to an abnormality of either monocytes or T lymphocytes. A similar decrease was seen in HCL and NHL. Experiments with mixtures of normal and CLL cells suggest that the decreased IL-6 production in CLL is due neither to an inhibition of normal blood mononuclear cells IL-6 production by B-CLL lymphocytes nor to the absence in CLL blood of cell(s) or cytokine(s) needed for IL-6 production.