Highly purified soybean lectin (SBL) was labeled with fluorescein isothiocyanate (FITC-SBL) or tritium (3H-SBL) and repurified by affinity chromatography. FITC-SBL was found to bind to living ceUls of 15 of the 22 Rhizobium japonicum strains tested. The lectin did not bind to cells of the other seven R. japonicum strains, or to cells of any of the nine Rhizobium strains tested which do not nodulate soybean. The binding of the lectin to the SBL-positive strains of R. japonicum was shown to be specific and reversible by hapten inhibition with Dgalactose or N-acetyl-D-galactosamine.The lectin-binding properties of the SBL-positive R. japonicum strains were found to change substantially with culture age. The percentage of ceUs in a population exhibiting fluorescence after exposure to FITC-SBL varied between 0 and 70%. The Intimate and specific symbiotic associations between leguminous plants and bacteria of the genus Rhizobium provide most of the biologically fixed nitrogen available for agriculture. The rhizobia enter root hairs of the host plant in a structure called the infection thread (4). The infection thread, which is believed to be a tubular, inward growing invagination of the host cell wall, carries the bacterial symbiont into the cortex of the root (11). There the bacteria enter the cytoplasm of host cell, surrounded by an envelope of host cell plasma membrane (12).Both the bacteria and cortical cells of the host proliferate to form a root nodule where nitrogen fixation takes place.The specificity of the Rhizobium-legume symbiosis is manifested by the failure of soil microorganisms other than rhizobia to gain effective entry into the plant by induction of infection thread structures, and also by host range specificity between members of the genus Rhizobium and the family Leguminosae. Dazzo and Hubbell (6), in a study of the symbiotic association between Rhizobium trifolii and white clover, have found that clover roots and R. trifolii possess a common antigen. This common antigen was shown to be present on the cell surfaces of infective strains of R. trifolii, but absent, inaccessible, or present in reduced quantities on noninfective strains. These authors isolated capsular polysaccharide material from infective R. trifolii which possessed cross-reactive antigenicity. They also provided evidence for the presence of a lectin in clover seed extracts capable of binding to isolated capsular antigen and to infective-but not the noninfective-strains of rhizobia. These results led Dazzo and Hubbell (6) to propose that the clover lectin provides a bridge between common antigen structures for the preferential adsorption of infective strains of R. trifolii to the root surface of the host.Wolpert and Albersheim (20) isolated lectins from four different legumes and obtained lipopolysaccharide preparations from four strains of Rhizobium, each capable of nodulating one of the legume species. The authors reported that the rhizobial lipopolysaccharides interacted with the host lectins, but not with the lectins ...