THE relationship of the plant-oncogenic properties of Agrobacterium tumefaciens to the presence in these strains of large plasmids (the Ti plasmids) has now been convincingly demonstrated in several ways: oncogenic strains cured of the Ti plasmid lose their oncogenicity irreversibly1–3; transfer of Ti plasmids to non-oncogenic plasmidless strains confers oncogenicity to these strains3–7. The first method used to transfer plasmids was based on observations made by Kerr8,9, who had developed a method for the transfer of oncogenicity before the association of oncogenicity with large plasmids was known. It was later demonstrated that this transfer of oncogenicity was due to the transfer of the Ti plasmid3,4. These observations suggested that in certain circumstances the Ti plasmids could behave as conjugative plasmids in promoting bacterial conjugation and plasmid transfer. This paper and its companion10 describe experiments confirming that suggestion, making use of the fact that the capacity of Agrobacterium strains to utilise the unusual amino acids octopine (N2-(D-1-carboxyethyl)-L-arginine) or nopaline N2-(1, 3-dicarboxypropyl)-L-arginine) is coded for by Ti plasmid linked genes3,5–7,11, and indicate that octopine and nopaline may act in inducing plasmid transfer.