The effects of two new semisynthetic glycosaminoglycans (GAGs), G871 and G872, on the crystal growth and agglomeration of calcium oxalate monohydrate (COM) were studied in artificial urine in vitro. A constant composition crystallization system and a seeded crystal growth system were used to measure the rate of crystal growth and degree of agglomeration. The zeta potential on the crystal surface was measured using a Coulter DELSA 440 doppler electrophoretic light scattering analyzer. The previously reported GAG analogue, sodium pentosan polysulphate (SPP or SP54), was studied as a reference inhibitor of crystallization using the same systems. All three substances had a concentration-dependent effect on the zeta potential and on the rate of crystal growth and agglomeration of COM in artificial urine. G872 had a significantly greater effect than either G871 or SPP on all the measured parameters. It is suggested that semisynthetic GAGs, such as those reported in this paper may be of use in the prevention of the recurrence of calcium oxalate stones.