The R21(TC) factor, obtained by transduction of the R10(TC.CM.SM.SA) factor with phage ε to group E Salmonella, is not transferable by the normal conjugal process. However, when R21(TC)+ transductants are infected with the F13 factor, the nontransferable R21(TC) factor acquires transmissibility by conjugation. R21(TC)+ conjugants of Escherichia coli K-12, to which only the R21(TC) factor was transmitted by cell-to-cell contact from an F′ R+ donor, were still unable to transfer their R21(TC) factor by conjugation. In crosses between Hfr and F−E. coli K-12 strains containing R21(TC), the gene responsible for tetracycline resistance was located on the E. coli K-12 chromosome between lac and pro, near lac.