Successful transplantation of autologous chondrocytes for repair of articular cartilage defects requires an undisturbed matrix-synthesis of the transplanted cells. This, in turn, is dependent on the composition of the synovial fluid (SF) of the respective joint. We addressed the question whether analysis of a patient's SF can predict the rate of matrix-synthesis of articular cartilage exposed to this SF in vitro. SF was obtained from 115 patients with disorders of the knee, including gonarthrosis (n = 44), meniscal tears (n = 10), rheumatoid arthritis (n = 53), and reactive arthritides (n = 8). In the SF, the following parameters were determined: Interleukin-1 beta, IL-6, IL-8, IL-1-RA, TNF alpha, Insulin-like growth-factor I (IGF-I), IGF-II, IGF-binding protein-2 (IGFBP-2), IGFBP-3 as well as total proteinase activity and total collagenase activity. To assess the effect of SF on the matrix synthesis of articular chondrocytes, bovine cartilage was incubated in the presence of SF, and the rate of proteoglycan synthesis subsequently determined. In some cases, a monoclonal antibody directed against IGF-I was added. SF from patients with OA or trauma, respectively, stimulated PG-synthesis of bovine cartilage more markedly than did SF from patients with rheumatic arthritides. On the average, 60 percent of the SF-induced increase of cartilage matrix synthesis could be titrated out by an anti-IGF-I-AB. The best predictor for the SF-effects on PG-synthesis of exposed cartilage was the proportion of free IGF-I (r = 0.573, p < 0.001, Spearman rank correlation) followed by the SF-concentrations of IGF-I (with a positive sign), IGFBP-3, IL-1 beta, and TNF alpha (all with a negative sign). According to our data, IGF-I is the most important anabolic factor in human SF with respect to cartilage PG-synthesis. The proportion of free IGF-I seems to be of special importance in this regard. Low SF-levels of free IGF-I could be identified as a possible risk-factor for a sub-optimal protoeglycan synthesis of chondrocytes exposed to this synovial milieu.