By the microbiological assay, utilizing Lactobacillus leichmannii as a test organism, variations of the vitamin B12 contents of various tissues in the course of development of chick embryo and the human fetus are observed. A comparison between the vitamin B12 contents in various tissues of pregnant rabbits are also made. (1)The vitamin B12 contents in liver, heart and lung of the chick embryo increased progressively from 12 days' incubation as the embryonic development proceeds