Screening method of agents against the R factor by the use of an Hfr made by integrative suppression with an R factor.

By the use of an Hfr strain made by integrative suppression of a derepressed R factor, R100-1 in a dnaA-ts mutant of Escherichia coli and several other appropriate control strains with and without the R factor, a screening system was established which can be used to detect agents which are either inhibitory to the replication of the R factor and lead to its "curing" or are preferentially inhibitory to the growth of the R-carrying bacteria. The validity of this system was shown with several agents already known to be experimentally effective.