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The presence of an ft sex factor can be detected by propagation of the I-specific phage Ifl. By use of this method of detection, a high proportion of strains with fi+ R factors were shown also to carry an ft factor which was frequently a second R factor. In some doubly R+ strains, the fi+ and the fh factor were observed to be transferred independently at conjugation.

Eight plasmids out of 26 tested confer partial serum resistance to Escherichia coli K-12 strains (Bhs character). Four Bhs+ plasmids were tested in a rough Salmonella typhimurium host, and three conferred partial serum resistance to this host as well. Plasmids representative of 19 different incompatibility groups were examined; the Bhs+ property was found in some (but not all) representatives of groups N, 0, S, T, W, and F'lac (FI), and there was no apparent association between other plasmid markers and Bhs. It is likely that more than one mechanism can promote the Bhs+ phenotype and that Bhs genes are responsible for modifications of the surface structures involved in serum sensitivity.

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