Hydrogen evolution from root nodules has been reported to make N2 fixation by some legume-Rhizobium symbiotic systems inefficient. We have surveyed the extent of H2 evolution and estimated relative efficiencies of nodules of Austrian winter peas formed by 15 strains of R. leguminosarum. Their rates of He evolution in air were about 30 ~ of the rates of H2 evolution under an atmosphere in which N2 was replaced by Ar. Relative efficiency values based on C2H2 reduction rates ranged from 0.55 to 0.80. With some of the strains, hydrogenase activities were demonstrated in intact nodules and in bacteroids, but the levels of activity were insufficient to recycle all the H2 evolved by the nitrogenase system. In both intact nodules and bacteroids the hydrogenase is less sensitive to 02 damage than the nitrogenase system, so H2 uptake capacity was observed in intact nodules by suppressing the nitrogenase-dependent H2 evolution with an atmosphere containing a high 02 concentration, and in bacteroids by using aerobically prepared bacteroid suspensions. The hydrogenase activity of both was dependent on 02 consumption. AKm for H2 of near 4 ~tM was determined in suspension of bacteroids from nodules formed by strains 128C53 and 128C56.