Lactose was fermented but not assimilated by the strain Bifidobacterium bifidum DSM 20082. The sugar uptake was measured with lactose C-14. K-m and V-max values were respectively 2.6 mM and 12.11 nmol/min/mg of cell protein. The lactose transport system and the beta-D-galactosidase were stimulated when the cells were grown with lactose, but isopropyl-beta-D-thiogalactopyranoside had no effect, Lactose uptake was inhibited by compounds which interfered with proton and metal ionophore, Na+, Li+, or K+ did not affect incorporation of lactose. Furthermore, the lactose uptake decreased when an inhibitor of ATP synthesis was used. From the results of this study, the strain contained an active lactose transport system, probably a proton symport as described for Escherichia coli but with a different regulation system.