Summary: At least one plasmid from each of the incompatibility groups B, C, F, H/S, Iα, Iδ, P, W and X was shown to be capable of transfer from K12 to EBF65/65. Transfer was influenced by the presence of pAV2 (thought to encode a restriction-modification system) in the recipient strain; however, not all plasmids belonging to a particular incompatibility group behaved identically. All plasmids were unstable to varying degrees in EBF65/65, but under suitable conditions were capable of transfer to further strains of EBF65/65 and re-transfer to K12. Of 40 recently isolated trimethoprim R plasmids 31 transferred successfully from K12 to EBF65/65, but 17 of these 31 required the introduction of a second mobilizing plasmid for re-transfer to occur.