1. A low urinary magnesium was induced in normal volunteer subjects by giving cellulose phosphate; magnesium was added in vitro to yield urine samples of normal and high magnesium concentrations
2. After rapid evaporation of these urine samples at pH 5.3 to standard osmolality the calcium oxalate crystals were measured by microscopy and isotopic methods
3. There was a clear inverse correlation between magnesium concentration and calcium oxalate crystal formation
4. The case for treating calcium oxalate urolithiasis with magnesium is strengthened.