Escherichia coli strain W adsorbs phage λ very efficiently but the phage does not form plaques on this strain because the DNA of λ is broken down in a majority of the infected cells shortly after adsorption. In a 10−3 to 10−4 fraction of the infected cells λ grows and small bursts of phage are produced. This phage does not carry the W-specific host modification and is unable to complete a second round of infection in W (Kerszman, Glover & Aronovitch, 1967). λω mutants have been isolated which are able to escape this restriction process and which plate on W with an efficiency of 1·0 and when grown in W these mutants carry a W-specific host modification.

Escherichia coli strain W was found to be lysogenic for a temperate phage Wφ. This phage, which plates on E. coli C, forms λ-like plaques 2–3 mm. diameter with turbid centres. It is serologically unrelated to λ but is closely related to P2 which it resembles in the electron microscope. Its buoyant density in CsCl has been measured and it is different from λ but similar to P2. E. coli C made lysogenic for Wφ restricts the growth of λ, and elsewhere (Kerszman, Glover & Aronovitch, 1967) it has been shown that the DNA of phage λ is degraded shortly after infection of bacteria lysogenic for Wφ. A mutant of Wφ has been isolated which has lost the property of restricting the growth of λ.