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The symbiotic roles of rhizobia with legumes have been extensively made use of in the agricultural field. An unresolved question about the infection process is the nature of the Rhizobium enzymes that degrade the plant cell wall. For the bacteria in the plant root-hair wall, carboxymethylcellulase (EC 18.104.22.168) could be one of the key enzymes in the early symbiotic process. Electron microscopic immunogold labeling experiments were performed with ultrathin sections of Sinorhizobium fredii CCRC15769. The results preliminarily indicated that antigenic determinants of membrane-bound carboxymethylcellulase are exposed, at the outside of the cytoplasmic membrane, to the periplasmic space; immuno-gold particles with 10nm diameter were hardly ever observed in the cytoplasm and other spaces.