SUMMARY: Transfer of derivatives of the bacteriocinogenic plasmid pRL1JI into eight symbiotically defective strains of R. leguminosarum resulted in suppression of the mutant phenotype in four cases. These included non-infective, ineffective and temperature-sensitive ineffective phenotypes. However, in none of these strains was the defect suppressible after high frequency transfer of derivatives of two other bacteriocinogenic plasmids, pRL3JI or pRL4JI. Nodulation ability was co-transferred at high frequency (> 95%) with bacteriocin production by the plasmid pRL1JI. The other two plasmids, pRL3JI and pRL4JI, also mediated the transfer of nodulation ability but at much lower frequency (10-3 to 10-4 per plasmid transfer). Sometimes, transconjugants that had acquired nodulation ability after the transfer of derivatives of plasmids pRL3JI or pRL4JI acted subsequently as high frequency donors for nodulation ability in a manner that was apparently similar to strains containing pRL1JI.