The broad host-range non-conjugative IncQ plasmid RSF1010 was mobilised with 100% efficiency in membrane filter matings, both in E. coli K12 and P. aeruginosa PAO, by broad host-range conjugative IncP plasmids. No homology between RSF1010 and an IncP plasmid could be detected. In E. coli, IncIα and IncX plasmids, but not IncF, IncN or IncW plasmids, were also relatively efficient at mobilising RSF1010, while in P. aeruginosa, R91–5 (IncP-10) was highly efficient, but pMG5 (IncP-2) and FP2 (IncP-8) were very inefficient. IncP plasmids also mobilised several plasmids derived from RSF1010 for use as in vectors in in vitro recombination experiments very efficiently, and pSC101 quite efficiently: this reduces the level of biological containment possible with these plasmids.