OBJECTIVE To imitate the in vivo joint situation and to allow cell interactions, a co-culture system of human osteoarthritic chondrocytes and synovial fibroblasts from a single joint was established and characterized with or without stimulation by IL-1 beta. METHODS Culture settings included chondrocytes in alginate alone, synovial fibroblasts in monolayer alone and a co-culture of both. Proteoglycan (PG) synthesis was measured by 35S-incorporation, PG content by a dimethylmethylene blue assay, DNA content by a fluorometric assay, and prostaglandin-E2 and IL-1 beta release by ELISA. RESULTS In co-culture PG synthesis by chondrocytes was significantly reduced in the presence of IL-1 beta (1 ng/ml) compared to controls. PG content of chondrocyte cultures was reduced for controls and IL-1 beta treated co-cultures. Synovial fibroblasts in co-culture did not show significant change of PG synthesis or content when compared to cells in mono-cell culture. PG release into the medium was relatively high in co-cultures. IL-1 beta significantly decreased the proliferation rate of chondrocytes in co-cultures and slightly increased prostaglandin-E2 release. CONCLUSIONS Co-culturing of osteoarthritic chondrocytes and synovial fibroblasts from a single human joint allows interactions between both entities and may offer a useful tool to study the effects of mediators or new drugs under more in vivo like conditions compared to mono-cell cultures.