We studied the effect of a lectin isolated from seeds of the legume Vatairea macrocarpa on net H+ efflux in Rhizobium tropici, a bacterium capable of nodulating legume Phaseolus vulgaris. V. macrocarpa lectin (VML) was observed to temporarily stimulate the specific net H(+) efflux in R. tropici. When VML was present at 32 microg ml(-1), with or without 2 microM galactose (Gal), a specific net efflux >2.4 pM H+(min)(-1) mg dry biomass(-1) was induced. There was no detectable net H+ efflux when bovine serum albumin (16 microg ml(-1)) was tested. Addition of 16 microgVMLml(-1) resulted in a 700% increase of the extracellular Na+ concentration. The soluble proteins in the supernatant containing VML extract indicate a maximum immobilization of +/-10 microgVMLml(-1), with a minimum of 36,600 dimers or 8500 larger aggregates of VML binding in each bacterium. Our data suggest that VML activates Rhizobium as a bioenergetic substrate molecule, resulting in potential alterations of the external bacterial membrane.