1) A simple and reliable method was developed for assay of calcium oxalate crystal growth. A metastable solution of CaCl₂ (1mM) and sodium oxalate (0.2mM) in 0.05M barbiturateacetate buffer, pH 5.7, NaCl 0.15M was seeded with calcium oxalate monohydrate cystals (0.06mg/ml). Tubes (12ml) were stirred and, at 40 min., aliquots were removed and filtered through Millipore filters (0.22μm). The ¹⁴C radioactivity was determined in the filtrate, and the ratio of the count to control, preseeding, was used to estimate the fraction of oxalate that had been lost from the liquid phase and had entered the solid phase by combining during growth of the seed crystals. The validity of this method was confirmed by many experiments.
2) Non-dialysable, high molecular weight inhibitor (s) in human urine contribute about half or more than half of the calcium oxalate crystal growth inhibitory activity of whole urine.
3) Although magnesium is weaker than inorganic pyrophosphate and citrate at the concentrations usually present in urine, its potency increases greatly when its concentration increases.