During an epidemic, a strain of resistant to ampicillin, kanamycin, tetracycline and which was colicinogenic was isolated. The three drug-resistance determinants were transferred singly, in pairs or to sensitive K12 strains. When the recipient was an Hfr strain, it remained sensitive to F-specific phage f 2, so the transferred resistance factors can be classified as . The three resistance characters were always independently transduced by phage P 1-b, and remained transferable in subsequent conjugations. In contrast to closely related R factors or Col factors which cannot coexist stably in the same cell, the three R factors described here do not specifically interfere with one another. They are maintained together in the naturally occurring strain of and after successive transfer to K12. These results support the hypothesis that three independent resistance factors, R (Am), R (Km), R (Tc) and a Col factor coexist in and K 12 strains.


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