Bifidobacterium longum ATCC 15707 cell production was studied in MRS medium supplemented with whey permeate (MRS-WP) during free-cell batch fermentations and continuous immobilized-cell cultures. Very high populations were measured after 12 h batch cultures in MRS-WP medium controlled at pH 5.5 (1.7 +/- 0.5 x 10(10) cfu/ml), approximately 2-fold higher than in non-supplemented MRS. Our study showed that WP is a low-cost source of lactose and other components that can be used to increase bifidobacteria cell production in MRS medium. Continuous fermentation in MRS-WP of B. longum immobilized in gellan gum gel beads produced the highest cell concentrations in the effluent (4.9 +/- 0.9 x 10(9) cfu/ml) at a dilution rate (D) of 0.5 h(-1). However, maximal volumetric productivity (6.9 +/-0.4 x 10(9) cfu ml(-1) h(-1)) during continuous cultures was obtained at D = 2.0 h(-1), and was approximately 9.5-fold higher than during free-cell batch cultures at an optimal pH of 5.5 (7.2 x 10(8) cfu ml(-1) h(-1)).