Although being largely used for pathobiological models of cartilage diseases such as osteoarthritis (OA), human chondrocytes are still enigmatic cells, in as much as a large part of their secretome is unknown. We took advantage of the recent development of antibody-based microarrays to study multiple protein expression by human chondrocytes obtained from one healthy and five osteoarthritic joints, in unstimulated conditions or after stimulation by the proinflammatory cytokines interleukin-1 (IL-1) or tumour necrosis factor (TNF). The secretion media of chondrocytes were incubated with array membranes consisting of 79 antibodies directed against cytokines, chemokines, and angiogenic or growth factors. Several proteins were identified as new secretion products of chondrocytes, including the growth or angiogenic factors EGF, thrombopoietin, GDNF, NT-3 and -4, and PIGF, the chemokines ENA-78, MCP-2, IP-10, MIP-3alpha, NAP-2, PARC, and the cytokines MIF, IL-12, and IL-16. Most of the newly identified chemokines were increased intensely after stimulation by IL-1 or TNF, as for other proteins of the array, including GRO proteins, GM-CSF, IL-6, IL-8, MIP-1beta, GCP-2, and osteoprotegerin. The up-regulation by cytokines suggested that these proteins may participate in the destruction of cartilage and/or in the initiation of chemotactic events within the joint during OA. In conclusion, the microarray approach enabled to unveil part of an as yet unexplored chondrocyte secretome. Our findings demonstrated that chondrocytes were equipped with a proinflammatory arsenal of proteins which may play an important part in the pathogenesis of OA and/or its drift towards an inflammatory, rheumatoid phenotype. (C) 2004 Elsevier Inc. All rights reserved.