Further understanding of how the FinQ fertility inhibition system of the I-like plasmid R62 inhibits transfer of the sex factor F has been gained by the use of a genetic assay for individual transfer gene products. The technique involved construction of a series of Flac plasmids carrying suppressible mutations in individual transfer genes together with a FinQ inhibitor-insensitive traQ mutation. The transfer of the Flac double mutants from a strain carrying wild-type Fhis and R62 then indicated the effect of R62-encoded transfer inhibition on the expression of individual F transfer genes. During such inhibition the products of genes traJ, traA, traE, traB and traC were present in quantities sufficient to permit efficient F transfer, whereas the levels of the traF, traH, traG and traD gene products were so reduced as to limit F transfer. These findings and a failure to obtain recombination between traC and traQ mutations suggest that the R62 fertility inhibition system terminates transcription of the transfer operon between traC and traF.