The understanding of the transformation process in symbiotic root nodule cells is important in several respects. Firstly there is the exchange of chemical substances between the eukaryotic and prokaryotic partners and secondly the exchange of recognition signals between the two types of cell. Thirdly there is a change or rerouting of the endomembrane flow in the host cell to produce the new membrane type, the peribacteroid membrane and fourthly the change in the host cell plasmamembrane to facilitate exchange with neighbouring uninfected plant cells. Classically histochemical methods have been used to deduce the relationships between various subcellular components. Cell subfractionation studies play an everincreasing role in this field however, and thus this article reviews the strategic and methodological background contributing to the present state of knowledge in this area.