The relationship between the transfer systems of several plasmids was investigated, using as criteria complementation of a series of transfer-deficient Flac mutants, the efficiency of plating of F-specific phages, and identification of surface exclusion systems. The transfer systems of ColV2 and ColVBtrp were similar to that of F except for surface exclusion: ColVBtrp specified a surface exclusion system different from that of F, and ColV2 did not specify any detectable system. The transfer system of R1–19 was also similar to that of Flac, but the products of traA, traI and traJ were plasmid-specific. R1–19 determined the same surface exclusion system as ColVBtrp, and this was under the control of the transfer inhibitor. The transfer systems of ColIbdrd and Flac were unrelated. ColEl was transferred by Flac traI− mutants, but not by mutants in other cistrons.