The interaction between calcium and glycosaminoglycans (GAGs) was studied using a calcium ion-selective electrode. The Ca-binding capacity of GAGs involved 16% of total calcium in the presence of chondroitin sulphate and 28% in the presence of pentosan polysulphate. The action of GAGs on the nucleation of uric acid and sodium urate was examined and inhibitory effects were observed. The action of uric acid as a heterogeneous nucleant of calcium oxalate was studied, and considerable promotion of the heterogeneous nucleation of calcium oxalate by uric acid was found, which could be inhibited by the action of GAGs. From these summarised in vitro results, we conclude that uric can constitute an important risk factor for calcium oxalate urolithiasis through heterogeneous nucleation and the GAGs can play an important role as preventive agents.