A protein that specifically agglutinates Rhizobium meliloti, the alfalfa root nodule endosymbiont, has been purified from alfalfa seed. Material cross-reactive to antiserum prepared against the purified agglutinin is present in all alfalfa varieties that were tested but is absent in corn and other legumes not nodulated by Rhizobium meliloti. Studies with nonnodulating mutants of this microorganism incapable of binding to alfalfa roots suggest that the agglutinin is responsible for specific recognition between Rhizobium meliloti and alfalfa and that this recognition is an essential step in nodule formation.