In a colony of germfree rats 50 per cent of the males had urinary calculi composed of calcium citrate and calcium oxalate. Genetically closely related conventional animals on the same sterilized diet did not present a single case of stone formation. The tendency to calculus formation disappeared when germfree animals were contaminated with the intestinal flora from conventional rats.

The calculus formation can readily be explained by the high calcium, high citrate, and high pH of the urine. This pattern was changed to that of conventional rats when the germfree rats were infected with intestinal microorganisms.

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