Intraarticular injections of viable N, gonorrhoeae, killed N. gonorrhoeae or gonococcal lipopolysaccharide (LPS) in rabbits' knees caused an acute, polymorphonuclear synovitis with abscess formation 24-72 h after the injection. At 5-7 days, a mononuclear infiltration with synovial lining cell hyperplasia developed, which in some rabbits persisted for one month. Gonococcal LPS, in amounts of 5 micrograms or greater, always caused a marked synovitis indistinguishable from that produced by viable N. gonorrhoeae. Gonococcal outer membrane protein used as a control in these experiments caused no or minimal synovitis in concentrations 50-fold higher than those used in the LPS inoculation experiments. These studies should provide a model to investigate the role of LPS in the arthritis associated with gonococcal infection.