The dependence of the nitrogenase activity of Rhizobium meliloti on the strain peculiarities of the cultures, the composition of the media used, and the metabolites of legume tissue cultures was demonstrated by the acetylene method. The nitrogenase activity is significantly higher in R. vigna than in R. meliloti, under the same experimental conditions. Enrichment of the Murashige-Skoog medium with arabinose (25 mM), succinate (25 mM), glutamine (2 mM nitrogen), and yeast extract (0.1%) substantially stimulated the nitrogenase activity of a pure culture of R. vigna. The maximum nitrogenase activity on this medium was noted when metabolites of sweet clover tissue were introduced.