The levels of serum interleukin-6 (IL-6) were measured during the course of adjuvant arthritis (FAA) in male Lewis rats. In the course of the disease serum IL-6 levels increase with a clear correlation with morphologic disease signs. Additionally, the FAA-inducing antigen, heat-killed Mycobacterium tuberculosis, was found to be a strong inducer of IL-6 production by spleen cells in vitro. The effects of the anti-rheumatic drugs indometacin, dexamethasone, cyclophosphamide and cyclosporin A (CsA) on IL-6 levels during FAA were determined. Complete normalization of serum IL-6 levels was observed with dexamethasone, cyclophosphamide and CsA whereas indometacin only partly reduced serum IL-6 levels.