Investigations were made on the causes of and countermeasures for high blank phenomenon in the microbiological determination of vittamin B_<12> with Lactobacillus leichmannii. One of the main causes of the phenomenon seems to be the change of requirement for vitamin B_<12> due to the mutation of the test organism during the stock culture, since a considerable destruction of the vitamin was recognized in the liver extract added to the stock culture medium during storage. The addition of a large amount of vitamin B_<12> to inoculum medium also resulted in high blank due to carrying over of the vitamin. It is desirable to use liver extract as fresh as possible for the stock culture medium and to add less than 0.5 mμg/cc of the vitamin to the inoculum medium.