1. Young rats were fed on diets containing heated or unheated soya-bean flour.
2. Feeding unheated soya-bean flour decreased concentrations of vitamin B12 in liver, kidneys and blood serum, urinary excretion of sulphate and concentration of reduced glutathione in liver. Blood glutathione level and urinary excretion of methylmalonic acid and formimino-glutamic acid following loads of propionate and histidine, respectively, were increased.
3. Supplementing a diet of unheated soya-bean flour with vitamin B12, had no effect on the level of reduced glutathione in the liver. Adding methionine to this diet decreased excretion of methylmalonic acid and the level of glutathione in blood.
4. The activity of liver methyl-tetrahydrofolate: L-homocysteine S-methyltransferase was not affected by the nature of dietary soya-bean flour nor by supplementation with vitamin B12
5. It is concluded that heat-labile substances present in soya-bean flour induce metabolic changes which can bring about an increased requirement for vitamin B12, and a deficiency of sulphur-containing amino acids.