High levels of interleukin 6 (IL 6/B cell stimulatory factor-2) were detected in synovial fluids from the joints of patients with active rheumatoid arthritis (RA). The cells found in freshly isolated synovial fluid constitutively expressed IL6 mRNA. The synovial tissues obtained by joint biopsy were also found to produce IL6 in vitro. Immunohistochemical analysis demonstrated that CD2+ T cells as well as CD20+ blastoid B cells in the synovial tissues produce IL6. The data indicate that IL6 is generated constitutively in RA and its overproduction may explain the local as well as the generalized symptoms of RA, since IL6 can function as B cell growth and differentiation factor as well as hepatocyte-stimulating factor.