A mutant of TnA no longer recognizing immune plasmids had been isolated. The mutation is complemented in trans by a functional tnpR gene. The requirement for wild type tnpR gene product for the establishment of transposition immunity was confirmed by the use of a derivative of transposon Tn3 in which both the tnpA and the tnpR genes are partly deleted. This deleted Tn3 was shown to transpose onto an immune plasmid in the presence of a wild type tnpA gene but not in the presence of both tnpA and tnpR genes.