Rats fed 60 µg of carotene daily together with a diet containing yeast-b, -c or -d, without additional vitamin B12, stored significantly more vitamin A than did those on a diet containing yeast-a. The addition to the synthetic vitamin-mixture diet of an amount of niacin approximating that in the yeast diet (4.0 mg/100 gm of diet) did not influence carotene utilization. Rats receiving the B vitamins from the synthetic vitamin mixture without added vitamin B12 stored significantly more vitamin A than did those on the yeast-a diet. However, their storage was similar to that of the rats receiving the yeast-b, -c or -d diets. The utilization of carotene was significantly increased when vitamin B12 was added to the yeast-a or -b diets. There was no significant increase when this vitamin was added to the yeast-c diet. The addition of vitamin B12 (3.0 µg/100 gm diet) to the diet containing a mixture of synthetic vitamins did not increase carotene utilization. Rats receiving this diet stored significantly less vitamin A than did those fed the yeast-a or -b diet with added vitamin B12. The results of this investigation indicate that some factor (or factors) in addition to vitamin B12 influenced the utilization of carotene.