In the present experiment, further researches were made on the microbial assay of vitamin B_<12> with KCN-boiling method. Blood was heated for 30 min. with KCN solution (500μg/ml) in boiling water bath, and after subsequent treatment of metaphosphoric acid, a direct L. leichmannii assay was made with the extract thus obtained. Such a determination was proved to be possible and useful as a simple estimation method of vitamin B_<12> in human blood.
