Specialized transducing lambda phages, lambda oriR1, harboring DNA from the resistance plasmid R1drd-19 and its copy mutant pKN103 were isolated. From measurements of CCC-DNA content it is concluded that upon infection the phages can establish themselves as self-replicating plasmids in recA hosts lysogenic for lambda. It is thought that this bypassing of lambda immunity is due to the presence of the R1 origin of replication. The plasmids are sensitive to the incompatibility expressed by plasmid R1. This has been shown mainly by transduction of lambda oriR1 into recipients containing R1 plasmids or plasmid pBR322 carrying the basic replicon. We were able to demonstrate that a copy mutant of plasmid R1 was insensitive to copA+, but sensitive to the concerted action of PstI fragments F1 and F2. This mutant was previously assumed to be of dominant type. Physical mapping of the lambda oriR1 derivatives verified that they carry the basic replicon of plasmid R1. The plasmids are not stably maintained, but are lost in a frequency of 1%-2% per cell generation, which is consistent with their lack of the R1par region.