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Three rat hybridoma cell lines have been isolated which produce monoclonal antibodies identifying a noduleenhanced, soluble component of Pisum sativum root nodules. These antibodies each recognized a protease‐sensitive band (Mr 95K) on SDS‐polyacrylamide gels. The 95K antigen was resolved by isoelectric focusing into acidic and neutral components which were separately detected by AFRC MAC 236 and MAC 265 respectively. The third antibody (MAC 204) reacted with both acidic and neutral components through an epitope that was sensitive to periodate oxidation. These monoclonal antibodies were used for immunogold localizations at light and electron microscopic levels. In each case, the antigen was shown to be present in the matrix that surrounds the invading rhizobia in infection threads and infection droplets, as well as in the intercellular spaces between plant cell walls of nodules and also of uninfected roots. By contrast, a fourth monoclonal antibody, AFRC JIM 5, labelled a pectic component in the walls of infection threads, and JIM 5 was also found to label the middle lamella of plant cell walls, especially at three‐way junctions between cells. The composition and structure of the infection thread lumen is thus comparable to that of an intercellular space.

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Plasmids obtained from four field isolates of Rhizobium leguminosarum were visualized following electrophoresis on agarose gels. The relative mobilities of the bands 'observed corresponded to plasmids with molecular weights of about 1OOx lo6 and greater. Each field isolate examined had a different pattern of plasmids.Lysates from R. leguminosarum strain 300 normally produced three plasmid bands, although in some preparations two much larger plasmids were also visible. The smallest plasmid band seen in strain 300 probably contains two co-migrating plasmids, because in one derivative of strain 300 it was replaced by a doublet, presumably as a result of the presence of a small deletion in one of the co-migrating plasmids. No apparent symbiotic defects were associated with the presence of this deletion. However, a non-nodulating derivative of strain 300 (strain 6015) was found to have suffered a deletion in the thirdsmallest plasmid of this strain.Transfer of the determinant of medium-sized bacteriocin production pRL 1 JI (from isolate 248) was correlated with the appearance of an extra plasmid with a molecular weight of about 130 x lo6. Another determinant of medium bacteriocinogenicity, pRL4JI (from isolate 309), was correlated with the presence of an extra plasmid of the same size (about 160x lo6) as one seen in the donor strain 309. The third determinant, pRL3JI (from isolate 306), could not be correlated with the presence of an extra plasmid of the same size as any in strain 306, and it appears that recombination occurred between pRL3JI and plasmids resident in strain 300.