The effects of L-aspartic acid, alanine and L-glutamic acid on the crystallization of calcium oxalate were followed by two methods: measurements of the changes in calcium ion concentrations and determination of crystal size distribution in the calcium oxalate precipitate. The changes in calcium ion concentration were detected by a specific calcium electrode and are expressed as Discriminating Index (DI) values. Median size and number of crystals per milliliter were determined by Particle Data Counter. Glutamic acid in the range of 0–200 ppm was added to solutions of CaCl 2 which were subsequently mixed with solutions of Na 2C 2O 4 to yield CaC 2O 4 precipitation. At relatively high concentrations of glutamic acid, namely 20–200 ppm, the precipitation of calcium oxalate is retarded. However, at low concentrations, 2.5–20 ppm oxalate precipitation is enhanced. The two independent methods yield well-correlated results. Both L-aspartic acid and alanine did not affect the precipitation of calcium oxalate at similar experimental conditions.