A polyclonal antiserum generated against the Bradyrhizobium japonicum lectin BJ38 was characterized to be specifically directed against the protein. Treatment of B. japonicum cells with this antiserum and subsequent visualization with transmission electron microscopy and both conventional and confocal fluorescence microscopy revealed BJ38 at only one pole of the bacterium. BJ38 appeared to be organized in a tuft-like mass, separated from the bacterial outer membrane. BJ38 localization was coincident with the attachment site for (i) homotypic agglutination to other B. japonicum cells, (ii) adhesion to the cultured soybean cell line SB-1, and (iii) adsorption to Sepharose beads covalently derivatized with lactose. In contrast, the plant lectin soybean agglutinin labeled the bacteria at the pole distant from the bacterial attachment site. These results indicate that the topological distribution of BJ38 is consistent with a suggested role for this bacterial lectin in the polar binding of B. japonicum to other cells and surfaces.