A total of 2716 R factors and transfer factors isolated from Escherichia coli and salmonellas of human and animal origin were studied for their phage-restrictive effects in Salmonella typhimurium phage type 36. All of 1402 wild fi+ factors were non-restricting. The F factor of E. coli K12 was unique among the F-like factors tested in that it inhibited lysis of type 36 by one typing phage. In contrast, eleven distinct changes in the phage type of 36 were produced by fi− I-like factors. I-like plasmids can thus be subdivided by this method.
I-like R factors and transfer factors from human and animal enterobacteria were categorized by their phage-restrictive effects in type 36. Factors resembling Δ in this respect predominated among fi− I-like factor from human E. coli and S. typhimurium and from porcine E. coli. Δ-like and Coll-like fi− factors were equally distributed in bovine S. typhimurium. Coll-like factors were commonest in bovine and avian E. coli.