Kondo, Eiko (Gunma University, Maebashi, Japan), and Susumu Mitsuhashi. Drug resistance of enteric bacteria. IV. Active transducing phage P1 CM produced by the combination of R factor with phage P1. J. Bacteriol. 88:1266-1276. 1964.-During an investigation of the transduction of R factors with phage P1, a phage lysate capable of transducing the character of chloramphenicol resistance (CM(r)) in extremely high frequency was obtained. The transduction of the CM(r) character with the lysate was consistently accompanied by lysogenization with the phage used for transduction. This lysate exhibits no beneficial effect with normal P1, and no effect is produced by decreasing the multiplicity of infection. A single infection with the phage allows the formation of plaques as well as CM(r) lysogenic cells at the center of the plaque. Both the transducing and plaque-forming activities of the lysate were lost by neutralization with anti-P1 phage serum, and its absorption to the host bacteria was enhanced by the addition of Ca(++). Thus, it was concluded that a derivative of P1 phage (P1 CM) was isolated which had not only the ability to transduce the CM(r) character but also the capacity to form plaques; i.e., the CM(r) gene of R factor is specifically associated with the genome of phage P1. No detectable differences were noted between P1 CM and normal P1 phage in density-gradient analyses in CsCl, in stability of lysogenization, in ability to transduce chromosomal markers, and in the mode of induction from lysogenic cells by ultraviolet irradiation. The instance of transduction of the CM(r) character described here may also be considered as an example of lysogenic conversion, in the sense that the alteration in CM(r) character is inseparable from lysogenicity.