A hydrogenase has been purified to homogeneity from the soluble fraction of the rumen bacterium Megasphaera elsdenii, the overall purification is 200 times with a yield of 14%. The pure enzyme consists of a single polypeptide chain with Mr approximately 50 000 which contains 12 atoms of non-haem iron and 12 atoms of acid-labile sulphide. The enzyme is rapidly inactivated by O2 and it is therefore purified under nitrogen and in the presence of sodium dithionite. The optical spectrum of the enzyme, after removal of the dithionite with air, shows a peak at 275 nm (epsilon 275 nm = 143 mM-1 cm-1) and a shoulder between 350 nm and 400 nm (epsilon 400 nm = 46 mM-1 cm-1). The enzyme catalyses hydrogen production from sodium dithionite at a low rate. The rate is greatly enhanced by addition of the electron donors flavodoxin, ferredoxin and methyl viologen. The kinetic data with these three electron donors suggest co-operativity, but no indication of self-association of the enzyme was obtained. Sodium chloride enhances the rate of hydrogen production with methyl viologen semiquinone and changes the kinetic behaviour of the enzyme with this electron donor, but causes inhibition of the reactions mediated by ferredoxin and flavodoxin. Two kinetic models were developed which are consistent with the kinetic data of the three electron donors tested. The apparent co-operativity for the hydrogen production can be fitted with the mathematical form of those models. The identical kinetic behaviour of the hydrogenase with the one-electron donors flavodoxin and methyl viologen semiquinone monomer and the two-electron donor ferredoxin indicates that the hydrogenase accepts two electrons in two separate, independent steps and further indicates that the two (4Fe-4S) clusters of the donor ferredoxin are independent. The interpretation of the kinetic data with methyl viologen semiquinone is complicated by the fact that the semiquinone dimerises, and that the formation of the dimer is enhanced by salt. Taking into account the association of this donor, the activity of the enzyme with methyl viologen semiquinone can be described by the sum of the activities of the enzyme with methyl viologen monomer and methyl viologen dimer. The enzyme catalyses the oxidation of hydrogen gas with methyl and benzyl viologen as electron acceptors to their semiquinone forms; both electron acceptors show Michaelis-Menten kinetics. The hydrogen oxidation activity with both electron acceptors is stimulated by addition of sodium chloride. The kinetic data of the oxidation of hydrogen with the two-electron acceptors used are consistent with the porposed models, if it is assumed that the pathway followed is compulsory. At this moment no choice can be made between the models proposed.