Summary: Twenty-eight R factors transmissible to K12 were derived from 178 naturally occurring strains of and evidence was found of non-transmissible resistance plasmids in some strains. Nine plasmids were assigned to group N; one strain carried two N plasmids which were incompatible in reference strains of or Six R factors of group FII were found. One was unique among F-like plasmids in conferring II restriction. Entry exclusion by these R factors had a specificity distinct from that of FII plasmids previously described. Three R factors of compatibility group FI were the first recognized plasmids of this group with repressed pilus synthesis. Two R factors incompatible with R6K (and with this plasmid constituting group X) were identified, as were one plasmid of group P and an R factor compatible with plasmids of all groups so far described and therefore assigned to a new group (M). Similarities and differences between the R factor sets of and are discussed.


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