The rhizobium-soybean callus association in both symbiotic or asymbiolic system caused the R. japonicum to establish an acetylene reduction (nitrogen fixation) system. The symbiosis between rhizobium and callus was not an obligativc requirement for the induction of nitro-genasc synthesis in rhizobia. However, some unknown critical substance(s) produced by the callus is required to initiate the nitrogenasc synthesis in the rhizobium. The rhizobia in the symbiotic rhizobium-callus system had 10* times higher nitrogen fixing activity than those in the asymbiotic system. The development of nitrogenasc in rhizobium in both systems， unlike in intact plant, was not inhibited by the presence of combined nitrogen in medium. The nitrogen fixation might not be interferred by the nitrate assimilatory processes. The callus could regulate the nitrogen fixing activity of rhizobia but the level of rhizobial nitrogen fixation could not delay the senescence of callus tissues. The rhizobial nitrogen fixing activity declined at the time of callus tissues senesced.