1. Samples of fresh human urine were treated with immobilized uricase to lower urate concentration. Urate was added to yield low, normal and high urate samples.

2. Each sample was rapidly evaporated at pH 5.3 to standard osmolality and the yield of calcium oxalate crystals measured either by semi-quantitative microscopy or fully quantitative radioisotope techniques.

3. Increase of urinary urate did not increase the calcium oxalate crystals formed and may even have had an opposite effect.

4. Allantoin was without significant effect upon calcium oxalate crystal formation.

5. These data provide no support for the suggestion that reducing urate concentrations in the urine may be of value in treatment of patients with calcium oxalate stones.

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