This study analyzes the expression of monocyte chemoattractant protein-1 (MCP-1) by inflamed synovial tissue and defines its regulation in cultured synoviocytes. Synoviocytes from patients with rheumatoid arthritis and osteoarthritis express the 0.7-kb MCP-1 mRNA. Stimulation of synoviocytes with IL-1, TNF-alpha, LPS, platelet-derived growth factor, and transforming growth factor-beta-1, but not with basic fibroblast growth factor causes a marked increase in MCP-1 mRNA levels. Expression of the MCP-1 gene is inducible by activators of the protein kinase A (cAMP) and C (PMA) signal transduction pathways and is differentially regulated by the steroids dexamethasone and retinoic acid. Cultured synoviocytes de novo synthesize 12-, 15-, and 15.2-kDa MCP-1 proteins, which increase after stimulation with IL-1. Synovial tissues from donors without joint disease and from patients with rheumatoid or osteoarthritis were analyzed for MCP-1 mRNA expression by in situ hybridization. In these samples MCP-1 mRNA expressing cells were predominantly found in the sublining cell layers, whereas specimens of normal synovial tissue contained only few positive cells. These results identify synoviocytes as a source of MCP-1. Its expression is controlled by peptide regulatory factors that are known to be present in arthritic joints. Detection of cells producing MCP-1 mRNA in synovial tissues from patients with arthritis shows that this gene is expressed in vivo and suggests that MCP-1 can play a role in recruiting monocytes in joint inflammation.