Growth of Methanobacterium thermoautotrophicum on H2 and CO2 as sole energy and carbon sources was found to be dependent on Ni, Co, and Mo. At low concentrations of Ni (less than 100 nM), Co (less than 10 nM) and Mo (less than 10 nM) the amount of cells formed was roughly proportional to the amount of transition metal added to the medium; for the formation of 1 g cells (dry weight) approximately 150 nmol NiCl2, 20 nmol CoCl2 and 20 nmol Na2MoO4 were required. A dependence of growth on Cu, Mn, Zn, Ca, Al, and B could not be demonstrated. Conditions are described under which the bacterium grew exponentially with a doubling time of 1.8 h up to a cell density of 2 g cells (dry weight)/l.