F42lac recombination with lambda plac5 is normally twentyfold to fiftyfold higher than recombination between lambda plac5 and a chromosomal lac gene. The presence of an fi+ R1 plasmid in the same cell as F42lac dramatically reduces this enhanced recombination level while the fi- R1drd19 plasmid has little effect. When F42lac traJ90 is tested in a sup+ strain, it shows a sharp reduction in recombination with lambda plac5 that can be largely reversed by the presence of a supF mutation that partially suppresses the traJ90 nonsense mutation. It is concluded that the enhanced recombination between F42lac and lambda plac5 is largely dependent on the constitutive expression of F42lac fertility functions.