Nitrate reductase (NR) activity was detected in membranes from cells of Bradyrhizobium japonicum cultured in defined medium either with glutamate or nitrate as the only nitrogen source. With gel filtration, the relative molecular mass (Mr) of the NR in cells grown with glutamate was estimated to be about 78 kDa. The enzyme from cells grown aerobically with nitrate had an Mr of 236 kDa, the same as that of the NR from microaerobically nitrate-grown cells. When cells that had been grown with glutamate were incubated microaerobically in both the absence and the presence of nitrate, the enzyme from each source resembled that of nitrate-grown cells in having an Mr of 236 kDa. In glutamate-grown cells that were further incubated, both microaerobiosis and nitrate were required for fully expression of the activity of the enzyme.