Plasmid pRP761 is a derivative of the promiscuous plasmid RP4, which has a Tn 76 insert 1.8 kb from its EcoRI site within the trfB region (Barth 1979). This mutation was pleiotropic, having three effects: the plasmid is unstably maintained in E. coli, it reduces the growth rate of its host and it has suffered a reduction in host-range. We show that pRP761 has reduced expression from both its korA and korB genes and that Tn 76 has inserted between them. Fragment exchange experiments showed that this is the only mutant region in pRP761 and is therefore solely responsible for the pleiotropic effects. A spontaneous deletion derivative pRP761-6 has lost Tn 76 and its adjacent kilA and korA genes: it has reacquired stability, does not inhibit host growth but is still reduced in its host-range. The provision of cloned korA+ in trans complements the first two phenotypic effects in pRP761 to a large extent, but neither korA+ alone nor korA+ with korB+ complements the host-range reduction in pRP761 or pRP761-6. A possible explanation for these results is that there is a site between korA and korB, affected by the Tn 76 insert, that is essential to stable replication of these plasmids in some of their bacterial hosts.