A transfer set in particular for delivering a mixture of liquids for medical purposes, from a so-called flexible main bag containing a first liquid and from at least a so-called auxiliary bag containing a second liquid, connectable to the main bag through a pair of Luer-Lock connectors respectively male and female. Each of the two connectors has, at one end, a frangible section engaged in the end of a tubing connected to one of the bags and, at the other end, elements for being coupled to the other connector, provided with elements for locking the connectors in coupled position. The invention is useful for delivering liquids for medical purposes.