1. The organic matrices of 12 kidney stones containing calcium and two composed of uric acid were solubilized, with ethylenediaminetetra-acetate for the former and sodium hydroxide for the latter.

2. The solubilized matrices and residual fine particulate material were examined for sialic acid by the thiobarbituric acid method.

3. Sialic acid was found in every stone in either the soluble and/or insoluble material.

4. The identity of sialic acid was confirmed by the absorption spectrum of the colour produced and by its release from the protein by neuraminidase.

5. The presence of sialic acid in all stones despite widely varying composition suggests that it may be passively deposited.

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