1. Isoalloxazines were characterized and measured in the caecum of two conventional rabbits kept on an adequate diet, two rabbits on the same diet from which vitamin B2 (ribo-flavine) had been omitted and five germ-free rabbits which were also on the adequate diet.
2. Riboflavine (up to 12mg) unaccompanied by any of its degradation products was found in the caecum of germ-free rabbits and 8–9 μg per ml were found in their urine.
3. Hydroxyethylflavine (4–5 mg) accompanied only by traces of riboflavine was found in the caecum of conventional rabbits.
4. There was about 1% ash in the contents of each of five germ-free and two conventional caecums.
5. The five germ-free caecal contents averaged 22.4% of the animal's body-weight. For four conventional animals this figure was only 10.5%.
6. In the caecum of each of two conventional rabbits kept on a diet deficient in riboflavine neither riboflavine nor hydroxyethylflavine was found but there was somewhat less than 1 mg of a yellow-fluorescing material of low RF value in each caecum.
7. The results are discussed in relation to the authors' previous reports on metabolites of riboflavine.