Bradyrhizobium japonicum (R110d) exhibited four saccharide-specific binding activities: (a) adsorption to Sepharose beads containing covalently coupled lactose; (b) homotypic agglutination through one pole of the cell (star formation); (c) heterotypic adhesion to the cultured soybean cell line, SB-1; and (d) attachment to roots of soybean plants. Each of these binding activities can be inhibited by the addition of galactose or lactose, but not by derivatives such as N-acetyl-D-galactosamine or melibiose. Treatment of wild-type bacteria with N-methyl-N'-nitro-N-nitrosoguanidine followed by selection on the basis of reduced binding to SB-1 cells, resulted in two specific mutants, designated N4 and N6. Compared to wild type, these two mutants also exhibited decreased binding activity in: (a) adsorption to lactose-Sepharose beads; (b) homotypic star formation; and (c) heterotypic attachment to roots of soybeans plants. These results suggest that all four of the saccharide-inhibitable binding activities of Bradyrhizobium japonicum may be mediated by the same mechanism(s) or molecular component(s).