Bacteria undergo horizontal gene transfer via various mechanisms. We recently reported that cell-to-cell transfer of nonconjugative plasmids occurs between strains of Escherichia coli in co-cultures, and that a specific strain (CAG18439) causes frequent plasmid transfer involving a DNase-sensitive mechanism, which we termed "cell-to-cell transformation". Here we found that CAG18439 is a type of P1 bacteriophage lysogen that continuously releases phages. We tested the ability of P1vir bacteriophage to induce horizontal plasmid transfer and demonstrated that such a horizontal plasmid transfer was caused by adding culture supernatants of P1vir-infected cells harboring plasmids to other plasmid-free cells. This plasmid transfer system also reproduced the major features of plasmid transfer involving CAG18439, suggesting that P1vir-induced plasmid transfer is equivalent or very similar to plasmid transfer involving CAG18439. We further revealed that approximately two-thirds of the P1vir-induced plasmid transfer was DNase-sensitive, but that complete abolition of plasmid transfer was observed when proteins were denatured or removed, despite the presence or absence of DNase. Therefore, we concluded that P1vir-induced plasmid transfer is largely due to the occurrence of cell-to-cell transformation, which involves the assistance of some proteinaceous factor, and partly due to the occurrence of plasmid transduction, which is mediated by phage virions. This is the first demonstration of the P1-phage-induced cell-to-cell transformation.