Disease activity in rheumatoid arthritis (RA) is frequently associated with a marked acute phase response, thrombocytosis, hypergammaglobulinemia and the presence of rheumatoid factors (RF). Interleukin 6 (IL-6) is a cytokine that stimulates hepatocytes to produce acute phase reactants and B lymphocytes to produce immunoglobulin. We measured serum IL-6 in patients with active RA during induction of intramuscular gold therapy. Levels of IL-6 correlated strongly with the erythrocyte sedimentation rate and RF titers and to a lesser degree with platelet counts. It may, therefore, be responsible for many of the systemic manifestations of RA and its measurement in the serum may be a convenient and reproducible variable for the assessment of disease activity in this very common disease.