We investigated the cytokine network in rabbit lipopolysaccharide (LPS)-induced arthritis, using inhibitors against homologous TNF alpha, IL-1 beta, and IL-8. Rabbits were intraarticularly injected with LPS (10 ng) and cytokine inhibitors (10 micrograms each), and the concentrations of each cytokine in the synovial fluids were measured. Maximum levels of TNF alpha and IL-8 were detected at 2 hours after LPS-injection, whereas IL-1 beta and IL-1 receptor antagonist (IL-1Ra) were detected at 6 and 9 hours, respectively. By immunohistochemistry, synovial lining cells were positive for TNF alpha and IL-8, and infiltrating leukocytes were positive for IL-1 beta and IL-1 Ra. The effects of cytokine inhibitors on the release of each cytokine were then investigated. The maximum levels of TNF alpha and IL-8 were not affected by blocking the activities of other cytokines. In contrast, the peak concentration of IL-1 beta was reduced by anti-TNF alpha monoclonal Ab (mAb), IL-1 Ra or anti-IL-8 IgG. Peak concentrations of IL-1Ra were reduced by anti-TNF alpha mAb or anti-IL-8 IgG. Anti-TNF alpha mAb, IL-1Ra, and anti-IL-8 IgG reduced the recruitment of leukocytes into the joint cavity, and the effect of anti-IL-8 IgG was less than that of anti-TNF alpha mAb plus IL-1Ra. The initial phase of the leukocyte influx was not inhibited. These results provide new evidence that IL-8 as well as TNF alpha are the most proximal cytokines and induce subsequent production of IL-1 beta and IL-1Ra. The data also raise the possibility that factor(s) other than IL-8 may be involved in the leukocyte influx in LPS-induced arthritis.