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 IL 6 mRNA. The synovial tissues obtained by joint biopsy were also found to produce IL 6 in vitro. Immunohistochemical analysis demonstrated that CD2+ T cells as well as CD20+ blastoid B cells in the synovial tissues produce IL 6. The data indicate that IL 6 is generated constitutively in RA and its overproduction may explain the local as well as the generalized symptoms of RA, since IL 6 can function as B cell growth and differentiation factor as well as hepatocyte-stimulating factor.