Two-dimensional polyacrylamide gel electrophoresis of total proteins of Azospirillum brasilense Sp7 showed that the bacteria respond to the presence of plant root exudates by induction of some proteins. High-level induction of a 40-kDa acidic protein was demonstrated by Coomassie brilliant blue staining and Western blotting, using a specific polyclonal antiserum. This protein is also observed in A. lipoferum, but not in A. halopraeferens and A. irakense. In A. brasilense, the 40-kDa protein was induced by exudates from wheat, maize, bean and alfalfa. However, only maize exudates stimulated production of this protein in A. lipoferum.