Harada, Kenji (Gunma University, Maebashi, Japan), Mitsuo Kameda, Mitsue Suzuki, and Susumu Mitsuhashi. Drug resistance of enteric bacteria. III. Acquisition of transferability of nontransmissible R(TC) factor in cooperation with F factor and formation of FR(TC). J. Bacteriol. 88:1257-1265. 1964.-Transmissible drug-resistance factor R, which confers resistance to tetracycline, chloramphenicol, streptomycin, and sulfonamide, was previously found to be transduced in the system of the Salmonella E group with phage epsilon. The R factor of R(+) transductants was nontransmissible by cell-to-cell contact, and it was not eliminated by treatment with acridine dye. When R(+) transductants were infected with F factor, the nontransmissible R factor acquired transferability by conjugation. The R(+) conjugants, to which only the R factor was separately transmitted by conjugation from the (F(+)R(+)) donor, were still unable to transfer their R factor by conjugation. However, the (FR)(+) conjugants, to which both F and R factor were transmitted simultaneously by conjugation, were also capable of transferring their F and R factors by conjugation. From the present study, it was concluded that the recombinant (FR) factor was formed as a result of an interaction between F and R factors present in a host bacterium, and that one of the mechanisms of acquisition of transferability is accounted for by the formation of recombinant (FR) factor. The recombinant (FR) factor was transferable by conjugation, and it conferred both the drug-resistance and F(+) characters to the recipient cells. The (FR) factor was eliminated by treatment with acridine dye and also transduced as one unit into Escherichia coli K-12 by P1kc phage.