SUMMARYStrains of Salmonella typhimurium of predominantly Middle Eastern origin, but distributed from England to India, were found to carry at least three types of resistance plasmid. The most important was initially identified as an FI plasmid by compatibility tests, but differs from the F factor on the one hand and the FI factors R162 and ColV on the other. The

SUMMARYA 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 f-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 A in this respect predominated among fi-I-like factor from human E9. coli and S. typhimurium and from porcine E. coli. A-like and ColI-like f-factors were equally distributed in bovine S. typhimurium. ColI-like factors were commonest in bovine and avian E. coli.