The nitrogenase system in cell-free extracts of soybean nodule bacteroids was fractionated into 2 components by use of protamine sulfate or polypropylene glycol precipitation followed by chromatography on DEAE-cellulose. Iron and molybdenum were concentrated in 1 fraction and iron in the other. Combination of fractions resulted in a striking stimulation in activity relative to the activity of individual fractions. The effect of different proportions of the 2 fractions on specific activities was studied. The ratios of the rates of reduction of acetylene and N(2) by extracts or fractions of different purities were relatively constant. Extracts or fractions retained most of their nitrogenase activities when stored in liquid N(2).