The mode of response of a B₁₂-requiring organism, Lactobacills delbrueckii No. 1 to vitamin B₁₂ is analyzed. This organism requires 500 to 600 molecules of B₁₂ per cell for the normal cell multiplication. If B₁₂ is present in excess in a surrounding medium, the cell of this organism has an ability to take up and preserve as much as 8×10⁴ molecules of B₁₂ for the subsequent growth. On the other hand, when B₁₂ is deficient in a culture medium, cells elongate abnormally according to the degree of deficiency up to 300 to 500μ in length. In such cells, the number of B₁₂ molecules retaining per unit cell length (3μ) becomes only 5 to 6.
These distinctive features are found only in the B₁₂-requiring Lactobacillus and are discussed from the view of the regulation of cell growth.