Multiple sclerosis (MS) represents a T-cell-mediated autoimmune demyelinating disease of the central nervous system associated with altered immunoregulation. Interleukin (IL)-6 is a cytokine that has several effects on the neuroimmune system. Specific IL-6 receptors have been found in human lymphocytes and neuroglial cells. The aim of the present study was to assay IL-6 binding on peripheral blood T lymphocytes in MS patients. We found that T cells from MS patients had significantly more IL-6 receptors [Bmax: 279 +/- 7 vs. 246 +/- 8 (mean +/- SEM) receptors/cell, in patients and controls, respectively], whereas Kd values were similar to those of healthy subjects [26.8 +/- 0.7 vs. 25.4 +/- 0.6 (mean +/- SEM) pM, in patients and controls, respectively]. Significant (P < 0.05) differences in IL-6 binding values were observed between stable patients and those relapsing (272 +/- 9 vs. 300 +/- 12 (mean +/- SEM) receptors/cell, respectively). We found significantly (P < 0. 001) higher amounts of IL-6 receptors on CD4+ T cells from MS patients than on CD4+ lymphocytes from controls (434 +/- 11 vs. 363 +/- 9 (mean +/- SEM) receptors/cell, respectively); CD8+ T cells showed very few IL-6 receptors in both patients and controls. These data are discussed in terms of MS immune pathogenesis and pathophysiology, because T-cell activation seems to be linked to increased IL-6 binding. The upregulated IL-6 system might be involved in antibody-mediated demyelinating pathways, because IL-6 is well known to enhance humoral immune response.