Large F plasmids such as F'128 stimulate precise excision of the transposons Tn5 and Tn10 in E. coli K12. This stimulation occurs when the transposons are either on the F'128 plasmid or the bacterial chromosome. Stimulation of precise excision is dependent upon conjugal transfer proficient F'plasmids. Tra- mutations which are defective in conjugal transfer negate this F'128 plasmid stimulation effect. F'128 traS mutations, which are surface exclusion defective and thus permit matings between male cells, thereby increasing conjugal transfer, increase the F plasmid stimulation effect. When the F' plasmid is present in a cell with the small plasmid, pRS31, carrying the traS to traZ region of F, stimulation of precise excision is no longer observed. This complementation-like activity by pRS31 is abolished by a Tn5 insertion in the traS gene. Data are presented supporting the notion that F' plasmid stimulation of precise excision occurs in the recipient during conjugal transfer. F'128 traS also stimulates recA-dependent recombination between DNA sequences on the small, nontransferrable plasmid pRDK41, DNA sequences that are unrelated to those of the F plasmid. The F'plasmid stimulation of precise excision of Tn5 is not seen with F+ but only with certain F's with large insertions of chromosomal DNA.