Urine is supersaturated with respect to calcium oxalate (CaOx), which may lead to spontaneous nucleation of CaOx crystals. The CaOx concentration at which crystals begin to form is higher in urine than in simple salt solutions, implying the presence of nucleation inhibitors in urine. Osteopontin (OPN) is made by renal tubular cells, secreted into tubular fluid, and inhibits growth of CaOx crystals in vitro.’.* The concentration in rodent urine is 6-8 pg/ml; it has also been found in human urine. 3 OPN’s effect on crystal nucleation is unknown; because it is found in the matrix of Ca stones, it has been suggested that OPN may promote nucleation of CaOx.