Increased amounts of interleukin-8 (IL-8) were detected in synovial fluids of patients with active rheumatoid arthritis (RA) by radioimmunoassay (RIA). The concentration of IL-8 correlated directly with the number of infiltrating neutrophils in synovial fluids. To elucidate the role of IL-8 in neutrophil accumulation at the site of synovitis, the in vivo effects of intraarticular injection of recombinant IL-8 (rIL-8) on leukocytes infiltration into the joint space and synovium were examined. Following a single injection of rIL-8 into the knee joint space of rabbits, redness of the joint and limp became apparent after 4 h and were associated with the rapid infiltration of neutrophilic leukocytes into the joint space and synovial tissues. These effects were time dependent, first becoming evident at 1 h and reaching a plateau in 4 h, and also dose dependent, with a minimal effect being elicited by 100 ng per joint. Although neutrophils were present in the greatest number at 4 h, subsequently mononuclear cells accumulated and became apparent in considerable number after 8 h. Synovial lining cells became ovoid, pleomorphic, and multilayered at 24 h. IL-8 had no effect on the breakdown of proteoglycan of articular cartilage. Based on these findings, IL-8 released from monocytes and synovial cells may be an important contributor to leukocyte accumulation and inflammatory events in the joints of RA.