The existence of hydrogenase, the enzyme activating molecular H2, has been demonstrated in cultures of Azotobacter by a methylene blue reduction method as well as by measurements of H2 uptake in the Warburg microrespirometer. The enzyme is found in cells grown in the presence or absence of H2 and using either free or combined nitrogen. Cells of the pea organism taken directly from the nodule possess the enzyme but those grown on the usual laboratory media do not.