Collagen-induced arthritis was produced in rats by intradermal immunization with type II collagen and the expression and production of monocyte chemoattractant protein-1 (MCP-1) were examined by immunohistochemistry, enzyme-linked immunosorbent assay (ELISA), and Northern blot analysis. Two to three weeks after the immunization, the hindfeet showed swelling and redness, followed by the development of severe arthritis, particularly in the ankle joints. During this period, prominent infiltration of neutrophils and macrophages was observed. Sandwich ELISA and Northern blot analysis revealed that MCP-1 concentrations in the joint lavages and MCP-1 mRNA levels in the joint tissues both peaked at 2 weeks after the immunization. By immunohistochemistry, various types of cells, particularly neutrophils, macrophages, synovial cells, and vascular endothelial cells, stained positively for MCP-1. Finally, injection of a neutralizing monoclonal antibody against rat MCP-1 significantly decreased the number of exudate macrophages in the lesions and reduced the ankle swelling by about 30 per cent compared with controls. These results suggest that MCP-1 plays a critical role in this model in the recruitment of monocytes and in the development of arthritis.