Intramolecular recombinogenic recircularization (IRR) of linearized plasmid DNA was used to study mechanistic relationships between recombination functions in Escherichia coli in vivo. Homology requirement for IRR ranges from 1 to 11 bp, and does not exhibit any notable strain to strain variability, with recombination occurring at a large number of possible sites within the plasmid molecule. We show that recF- and recR-deficient strains exhibit greatly reduced IRR efficiency, although neither gene product is totally essential. Mutation of recF and recR does not alter the distribution of recombination sites nor the range of molecules produced during IRR. A recO-deficient strain did not exhibit dramatic reduction in efficiency of IRR, implying that RecF and RecR proteins maintain function during this mechanism in the absence of functional RecO. The main IRR mechanism is ruvA-, ruvB- and recG-dependent and there is a lower efficiency second IRR mechanism operating in ruvA, ruvB and recG mutants. Some evidence suggests that this second mechanism involves functions associated with the replisome.