Abstract: The complementary functioning of cross components of nitrogenases of the blue-green alga Anabaena azotica (HB 686) and of the strictly aerobic bacterium Azotobacter vinelandii wasstudied. Preliminary results showed that the addition of Fe-protein from Azotobacter vinelandiito the cell-free crude extract of Anabaena azotica, in which the nitrogenase activity is very lowor even barely detectable, may promote the nitrogenase activity of the latter. Component Ⅰ(Mo-Fe-protein) of A. azotica when crossed with Component Ⅱ(Fe-protein)of A. vinelandii gives a positive nitrogenase reaction, although the reciprocal cross does not.The Fe-protein of nitrogenase of A. azotica is highly labile and may become inactive even justafter being separated. The cross reaction between the nitrogenase of blue-green alga and aerobicAzotobacter is reported here for the first time. This finding implies that the functions ofthe components of nitrogenases for different kinds of nitrogen-fixing organisms at variousevolutionary levels may be quite similar.