Px, a hybrid between the serologically unrelated and heteroimmune <em>Salmonella</em> bacteriophages P22 and Py : I. Some properties of Py and Px, genetic evidence for the hybrid nature of Px, and phenotypic mixing between P22, Py and Px.

Two new temperate Salmonella bacteriophages, Py and Px are described. Py is serologically unrelated to P22 and heteroimmune to it. Px is shown to be a hybrid phage containing genetic material from the c-region of P22 and the host range gene of Py. Px is heteroimmune to Py. Px phages seem to be formed only during vegetative replication of P22 in Py lysogenic cells. Independent events of hybrid formation may lead to genetically different types, called Px variants. The Px variants are sensitive to the P22 repressor whereas P22 is not repressed in Px lysogenic cells. The latent periods of P22 and the variant Px1 are about 30 min, whereas Py has a latent period of 60 min. Px1 recombines with P22 or Py only within the regions of genetic homology. Phenotypic mixing between P22 and Px or Py yields in different frequencies all possible types of mixed particles except Py-DNA with a P22 protein coat