R factors of the compatibility class P were transferred between Escherichia coli K12 and Rhizobium leguminosarum. These R factors were stable in R. leguminosarum and conferred similar levels of antibiotic resistance to those in the corresponding R+ E. coli K12 hosts, with the exception of carbenicillin resistance, which was greatly reduced. Transfer between R. leguminosarum strains occurred by conjugation and was stimulated by conditions favoring spheroplast formation. R factor-mediated recombination could not be demonstrated. Rhizobium leguminosarum is a promising species for genetic studies of symbiotic nitrogen fixation due to its association with the pea, a well-known genetically model organism. However, no suitable method of genetic analysis exists for R. leguminosarum. The R factor transfer experiments were initiated to determine the ability of these bacteria to transfer DNA by conjugation and to demonstrate R factor-mediated recombination. R factors from the compatibility class P were chosen because they are transferable between different bacterial genera. The R factors were transferred between E. coli and R. leguminosarum in liquid medium crosses. The transfer was stimulated by the presence of NB or glycine in the mating medium. The frequency of transfer was highest at around 40-50% NB. The transfer was also observed in plate crosses, where R factors were selected for transfer using kanamycin. The transfer of R factors was not inhibited by the presence of DNase, suggesting that the transfer was not due to spontaneous transformation. The R factors were stable in R. leguminosarum, and no separation of resistance determinants or loss of R factors was observed in repeated subculturing. The transfer of R factors between R. leguminosarum strains was at a similar level to that of repressed R factors in the Enterobacteriaceae, suggesting that chromosomal mobilization might be too infrequent to be observed. The inability to demonstrate R factor-mediated recombination was not unexpected, as the frequency of recombinant formation is much lower than that of R factor transfer. The results show that R. leguminosarum can act as a recipient or donor of P group R factors in mixed culture with other bacteria. Conjugation is the most likely mode of transfer, as the transfer is not sensitive to DNase and can occur both to and from E. coli. The transfer of R factors between R. leguminosarum strains was stimulated by the presence of NB or glycine, which induce spheroplast formation and interfere with cell division. Future studies of conjugation in R. leguminosarum would benefit from searching for self-transmissible plasmids that might already be present in Rhizobium strains.R factors of the compatibility class P were transferred between Escherichia coli K12 and Rhizobium leguminosarum. These R factors were stable in R. leguminosarum and conferred similar levels of antibiotic resistance to those in the corresponding R+ E. coli K12 hosts, with the exception of carbenicillin resistance, which was greatly reduced. Transfer between R. leguminosarum strains occurred by conjugation and was stimulated by conditions favoring spheroplast formation. R factor-mediated recombination could not be demonstrated. Rhizobium leguminosarum is a promising species for genetic studies of symbiotic nitrogen fixation due to its association with the pea, a well-known genetically model organism. However, no suitable method of genetic analysis exists for R. leguminosarum. The R factor transfer experiments were initiated to determine the ability of these bacteria to transfer DNA by conjugation and to demonstrate R factor-mediated recombination. R factors from the compatibility class P were chosen because they are transferable between different bacterial genera. The R factors were transferred between E. coli and R. leguminosarum in liquid medium crosses. The transfer was stimulated by the presence of NB or glycine in the mating medium. The frequency of transfer was highest at around 40-50% NB. The transfer was also observed in plate crosses, where R factors were selected for transfer using kanamycin. The transfer of R factors was not inhibited by the presence of DNase, suggesting that the transfer was not due to spontaneous transformation. The R factors were stable in R. leguminosarum, and no separation of resistance determinants or loss of R factors was observed in repeated subculturing. The transfer of R factors between R. leguminosarum strains was at a similar level to that of repressed R factors in the Enterobacteriaceae, suggesting that chromosomal mobilization might be too infrequent to be observed. The inability to demonstrate R factor-mediated recombination was not unexpected, as the frequency of recombinant formation is much lower than that of R factor transfer. The results show that R. leguminosarum can act as a recipient or donor of P group R factors in mixed culture with other bacteria. Conjugation is the most likely mode of transfer, as the transfer is not sensitive to DNase and can occur both to and from E. coli. The transfer of R factors between R. leguminosarum strains was stimulated by the presence of NB or glycine, which induce spheroplast formation and interfere with cell division. Future studies of conjugation in R. leguminosarum would benefit from searching for self-transmissible plasmids that might already be present in Rhizobium strains.