A monoclonal antibody against the interleukin-6 receptor (IL-6R) has been used in a high-sensitivity immunofluorescence technique to study receptor expression on unstimulated blood lymphocytes. Most CD4 cells express IL-6 receptor, whilst a small and variable proportion of CD8 and B cells are positive. CD4+ cells express higher levels of receptor than CD8+ T cells, and CD45RO+ cells express higher levels than CD45RA cells.