mAb and polyclonal antibodies against human IL-6R were prepared by using a murine transfectant cell line expressing the human IL-6R and a synthetic oligopeptide made on the basis of the deduced amino acid sequence as immunogens. Immunoprecipitation of radiolabeled IL-6R with these antibodies showed that the Mr of a mature IL-6R was 80 kDa and its value was reduced to 50K after treatment with O- and N-glycanase and neuraminidase, indicating that IL-6R is a glycoprotein. Two mAb recognizing different epitopes were prepared. One, PM1 inhibited the binding of 125I-IL-6 to the receptor and blocked the IL-6-dependent growth of a T lymphoma line, KT3. PM1 could not bind to IL-6R when it was saturated with IL-6, indicating that this antibody recognizes the IL-6 binding or the adjacent site on IL-6R. The other, MT18 was not inhibited by IL-6 for its recognition of IL-6R, therefore, this could be used for cytofluorometric staining of normal cells. Nonstimulated B cells expressed undetectable amount of IL-6R regardless of the expression of surface IgD. However, after the stimulation with PWM, IL-6R was observed on IgD- B cells with a relatively large size, but subtly on IgD- small B cells and not on IgD+ B cells, fitting the function of IL-6 which acts on activated B cells to induce Ig production. In contrast, IL-6R was detected on non-stimulated CD4+/CD8- and CD4-/CD8+ T cells. The level of IL-6R on both T cell subpopulations was not significantly changed after stimulation with phytohemagglutinin.