A consideration of the factors which might be responsible for adaptation of strains of legume nodule bacteria to species of legumes suggests that the chemical complex of the hosts may be responsible. Similar relationships between the physiological complex of plants and their relation to invasion and disease production have been noted in plant parasites. The protein characteristics of 29 species of commonly cultivated legumes were studied by means of precipitin and anaphylaxis reactions. Results show that all members of any cross inoculation group are closely related with respect to the protein characteristics of their seed, and in the majority of cases all legumes with closely related seed protein complexes cross inoculate. The latter point, however, is not true in all cases.