Abstract To study the effect of synthetic lipid A on the expression of the adhesion molecule CD44 in synovium and cartilage, synthetic lipid A (10 μg/ml, 0.5 ml) diluted in 0.025% triethylamine (TEA) was injected into the left knee of male Wistar rats. The equivalent volume of 0.025% TEA was injected simultaneously into the right knee of the same rat as a control. The numbers of infiltrating neutrophils and mononuclear cells in the synovium were counted, and the expression of CD44 was detected immunohistochemically. Infiltration of neutrophils in the synovium of the knee reached a maximum at 12 h after intraarticular injection of lipid A (78.4 ± 5.6 cells/400× field), and had subsided at 7 days after injection (2.1 ± 0.6). CD44 expression in the lining cells of the synovium was detected 24 h after injection (2.5 ± 0.3 cell layers), and it lasted 7 days after injection (2.6 ± 0.4 cell layers). CD44 expression in cartilage started at 24 h (69.0% ± 4.6% positive area of specimen) and lasted 7 days after injection (27.3% ± 3.3%). Intraarticular injection of lipid A was proven to induce acute arthritis and CD44 expression in the synovial lining layers and articular cartilage.