Somatic antigen groups and minimal somatic antigen constitution of 28 strains ofRhizobium japonicumwere determined on the basis of their somatic cross-reactions and agglutinin absorptions. A total of 17 serological groups and 24 antigens was proposed to account for the reactions obtained. Some groups consisted of a complex of three or four strains while others comprised only one strain. Still others were composed of strains which were common to two serological groups. The antigenic complexity of rhizobial cells and the possibility of using "key-antisera" and polyvalent mixtures of "key-antisera" are discussed in relation to the classification of large numbers of field or greenhouse nodule isolates inRhizobiumcompetition studies.