The transference by conjugation of protease genetic information between Proteus mirabilis strains only occurs upon mobilization by a conjugative plasmid such as RP4 (Inc P group). Upon receiving the RP4 plasmid, the level of proteolytic activity of the protease-excreting P. mirabilis is reduced to about 50%. A similar phenomenon occurs when the protease character is mobilized by the RP4 plasmid from the above transconjugant to a non-protease-excreting recipient strain. The molecular mechanism underlying the interference of R plasmids with proteolytic activity remains to be elucidated but there is evidence suggesting that some alteration in the bacterial envelope might be involved.