The task of this work was the establishment of an effective transfer system for F'-plasmids from Escherichia coli to Proteus mirabilis. It is shown that cells of PG VI act as recipients in crosses with E. coli F' strains but with a low transfer rate of the plasmid. The presumption that a restriction -- modification system in P. mirabilis was the only reason for the low transfer could not be confirmed. An indirect selection method was developed to isolate P. mirabilis cells which are better recipients. Conjugation experiments showed that the isolated mutants had a better recipient capacity (increase of about 100). This is true not only for the transfer of a F'-plasmid but also for a R-plasmid. The stability of these plasmids in the mutant cells, however, was much lower than in the wild type.