Isolation of a bacteriophage specific for the F plus and Hfr mating types of Escherichia coli K-12.

Seven phage stocks have been prepared from a sample of sewerage. The stocks grow only on the F+ and Hfr mating types of Escherichia coli strain K-12. This specificity is not shared by any previously characterized phage which was tested. The specificity of one of the isolates, f1, has been well correlated with the salmonella cross-streak test described by Zinder.