We studied the effect of cyclosporine A, prednisolone, and the Ca2+ channel blocker verapamil on interleukin-6 binding to mitogen-activated peripheral blood mononuclear cells, using a flow cytometric technique and phycoerythrin-conjugated IL-6. All mitogenic stimuli up-regulated IL-6 binding to a variable degree. PHA alone or in combination with PMA was the most effective stimulant in up-regulating IL-6 binding in all the experiments performed. The main changes in IL-6 binding were seen in the large cell cluster, which consisted mainly of lymphoblasts. PHA and PHA/PMA, however, also up-regulated the mean fluorescence intensity on the small cell cluster, which consisted mainly of quiescent lymphocytes. The overall effect of the three pharmacological agents on mitogen-up-regulated IL-6 binding was minimal; most significant were a down-regulation by all three agents of IL-6 binding by small lymphocytes in PHA/PMA cultures, a down-regulation of IL-6 binding by CsA in PHA/PMA-induced large PBMC, and an up-regulation by verapamil of PMA-induced IL-6 binding in large PBMC. Measurements of IL-2 binding and of IL-6 production in the same cultures showed a different pattern than that seen with IL-6 binding, as well as different CsA, prednisolone, and verapamil action. In conclusion, by using a new flow cytometric technique providing information both about the quantity of bound cytokine and about the proportion of IL-6-binding cells, we have demonstrated that IL-6 receptor expression in vitro by PBMC can be up-regulated by the use of stimulants differing in the signal transduction pathways they activate. In addition, by using different pharmacological agents and stimuli to dissect different activation pathways of the in vitro immune response, we conclude that IL-6R generation is regulated differently from IL-6 production. Furthermore, since CsA and prednisolone are known inhibitors of in vitro IL-2 production, our results indicate that IL-6R generation does not rely exclusively on the presence of IL-2.