AbstractErythrocytes and different strains of plant pathogenic bacteria agglutinated with aqueous and saline 1M NaCl extracts from Phaseolus vulgaris L. seeds, which showed similar gel electrophoretic patterns of proteins.Haemagglutinating activity in aqueous extracts was inhibited by some carbohydrates, especially N‐acetyl‐D‐galactosamine, D‐galactose and lactose. The agglutinin in this extract was purified by utilizing its ability to bind to Pseudomonas pisi cells followed by its removal from the bacteria by 0.3M galactose.Mitogenic activity of purified agglutinin was calculated as incorporation of 125I‐iododesoxy‐uridine into DNA of lymphocytes in vitro, which gave values between 25 and 50 μg/ml. The molecular weight of the subunits was found to be 31,500, estimated by mobility in SDS‐PAGE.A solid phase competition‐binding radioimmunoassay for bean agglutinin was developed in order to determine the affinity of this lectin in bean plants to phytopathogenic bacteria. The highest level of affinity was associated with the system formed by, Ps. pisi and lectin from fresh seeds.