An in vitro mineralizing system using bovine achilles tendon developed by Thomas and Tomita (3) was modified to enable quantitative evaluation of mineralization. Using this modified method, the potential effect of various ions on the rate of calcium uptake from inorganic mineralization medium was measured. Of the elements tested, only silicon and fluoride accelerated calcium uptake, whereas magnesium had an inhibitory effect. The simultaneous presence of silicon and fluoride in the medium had a synergistic action on calcium uptake. Urine of stone formers showed high propensity to mineralize tendon collagen, but not the urine of non-stone formers. Total content, and concentration of silicon in urine of stone formers was significantly higher than in normal urine. Addition of silicon to non-stone formers urine enhanced its capacity to mineralize collagen in vitro. These results strongly suggest the possible involvement of silicon and fluoride in the genesis of urinary calculi in man.