The capacity of utilizing or of producing molecular hydrogen is widespread among microorganisms. An indication that a specific enzyme system might be concerned in the metabolism of hydrogen came from the demonstration by Tauss and Donath (1930) that a bacterial culture grown autotrophically on H2, 02, and C02 reduced methylene blue in the presence of molecular hydrogen. Subsequently, Stephenson and Stickland (1931) showed that a number of heterotrophic bacteria, mainly from the enteric group, were able to catalyze the reduction of methylene blue, fumarate, nitrate, and oxygen by means of molecular hydrogen. The activation of molecular hydrogen was considered to be mediated by an enzyme which was named "hydrogenase." Hydrogenase, as measured by catalytic hydrogenations of such substances as methylene blue,