Immunodiffusion reactions were studied with seven strains of Rhizobium japonicum and three strains of the cowpea miscellany by using antisera against eight of the strains. Most strains yielded only weak precipitin bands when untreated cell suspensions were used as antigens in the diffusions. Ultrasonic disruption or heat treatment of the cells led to stronger bands, and immersion in boiling water for 20 min was used as the standard procedure for preparing these bacteria for immunodiffusion analysis. Heat-labile antigens were detected in only a few strains; the major antigens of all of the strains appeared to be heat-stable. Many of the strains cross-reacted, sometimes in a nonreciprocal manner; unheated cell suspensions cross-reacted more widely but more weakly than the heated suspensions. Heat-treated crushed nodule preparations reacted well in immunodiffusions. The antigens of cultured cell and nodule extract (bacteroid) forms of three strains were compared. In one of these strains, an antigen present in the cultured cells was absent from the bacteroids. Unknown strains present in soybean root nodules were readily identified by immunodiffusion.