Serol. re-actions of root-nodule bacteria have been studied by several workers, but there seems to have been no detailed analysis which distinguished between flagellar and somatic agglutination and used the absorption technique for antigenic identity. This paper reports on the application of these techniques to a number of strains of R. meliloti obtained from several spp. of plants and widespread localities. It was; necessary to postulate at least 3 flagellar and 7 somatic antigens to explain the results obtained with 6 organisms; only 2 cultures appeared to be identical (at least quali-tatively). Flagellar antigens gave division into much less specific groups than somatic; this was shown both in a detailed study of the 6 strains, for which sera were avail-able, and in a more general examination of a further 42 cultures. Many of the latter had no antigen in common with any of the 7 somatic antigens postulated; only 1 strain failed to react with the flagellar antibodies. This analysis emphasized the heterogeneous nature of the species-at least 12 broad groupings could be recognized-yet i of all strains fell into the one group and this contained 9 out of 12 of the organisms isolated from plants of Medicago hispida var. denticulata which had been collected over widely separated areas of Australia.