The lactococci represent a group of micro-organisms of major importance for dairy product fermentations. Since dairying has changed to a large scale industry an increased demand exists for genetically stable strains with defined properties. This has given rise to the nesessity to render these organisms accessible to genetic manipulation and, as a consequence, the requirement for the availability of efficient gene transfer systems. The work described in this thesis was conducted in the framework of these requirements, and, consequently, deals with efforts to develop new reliable DNA transfer systems, to improve existing systems and to study parameters affecting DNA transfer in lactococci. ... Zie: Summar