The Azotobacter vinelandii nifBfdxNnifOQ operon is required for synthesis of the nitrogenase iron-molybdenum cofactor. To further characterize the roles of its gene products, specific antibodies against NifB and NifO were generated, and the NifB, NifO and NifQ gene products were visualized and identified in nitrogen-fixing A. vinelandii cell extracts by a combination of two-dimensional gel electrophoresis of radiolabelled extracts and immunological detection methods. The three proteins showed apparent pI and M(r) values similar to those expected from sequence data, except for NifO, which showed an apparent M(r) of ca. 23 kDa (vs. 16 kDa expected).