A simple method, based upon the separation of cellular proteins by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, has been devised for distinguishing between isolates of Rhizobium japonicum. Eleven laboratory strains, previously classified into five serogroups, were analyzed by gel electrophoresis. Groups determined subjectively according to protein patterns matched the serogroups, with one exception. Most strains within serogroups could be distinguished from one another. For studying the ecology of Rhizobium, an important advantage of this technique compared with serology or phage typing is that it discriminates among previously unencountered indigenous bacterial isolates as well as among known laboratory strains. SDS-gels were used to analyze the Rhizobium population of 500 nodules, sampled throughout the growing season, from soybeans at two different Wisconsin localities. Although the soybeans had been inoculated with laboratory strains of R. japonicum, indigenous R. japonicum predominated. At one location, 19 indigenous gel types were distinguished and classified mainly into four groups. At the other location, 18 gel types, falling mainly into three groups, were detected. The predominance of a particular group varied, in some cases dramatically, depending upon the time and depth of nodule formation
