The interrelationship between an F prime and an F-like or I-like R factor was studied in Escherichia coli K 12 strains carrying both. The donorability of such strains suggested that these plasmids were transferred independently by their own transfer systems. Plaque formation by several male- and female-specific phage was tested; the inhibition by Flac of several female-specific phage was not affected by an F-like R factor. Neither did an R factor affect the incompatibility between Flac and Fhis. However, the surface exclusion characteristic of Flac was lost in the presence of an F-like R factor. This is probably closely related to the simultaneous loss of donorability, the F-pilus, and the f+ antigen: all may be the result of the inhibition by an R product of the formation or activity of a single F product.