Recently it was found that the specific activity of H2-forming methylenetetrahydromethanopterin dehydrogenase (Hmd) in Methanothermobacter marburgensis (formerly Methanobacterium thermoautotrophicum strain Marburg) increased six-fold when the hydrogenotrophic archaeon was grown in chemostat culture under nickel-limited conditions. We report here that the increase is due, at least in part, to increased expression of the hmd gene. This was demonstrated by Northern and Western blot analysis. These techniques were also used to show that hmd expression in growing M. marburgensis is not under the control of the H2 concentration. Studies with monoclonal antibodies on the effect of growth conditions on the expression of hmdII and hmdIII, which have been proposed to encode Hmd isoenzymes, were also carried out. The results indicate that the expression of these two genes is regulated by H2 rather than by nickel, and that HmdIII and HmdIII most probably do not exhibit Hmd activity.