A process in which conjugation is used to transfer a plasmid vector to a target cell when the plasmid vector is not self-mobilizable. When (1) cells that have a plasmid with conjugative and mobilising functions are mixed with (2) cells that carry the plasmid vector and (3) target cells, mobilising plasmids enter the cells with the plasmid vector and mobilise the plasmid vector to enter into the target cells. Following tripartite mating, the target cells with the plasmid vector are separated from the other cell types in the mixture by various selection procedures.