SUMMARYA fermented milk with typical yoghurt flavour and aroma was prepared using a human strain ofLactobacillus acidophilus. Growth of the lactobacillus was stimulated when the whey proteins of the ultrafiltered skim-milk substrate were increased by addition of ultrafiltered Cheddar cheese whey. Fermentation, which was carried out at 37 °C, was accelerated by addition of lactase. The resulting product had a smooth consistency, a pH of 4⋅2, acetaldehyde levels up to 15 p.p.m. and no off-flavours.L. acidophilusremained viable at a level of approx. 108colony forming units/ml, with no impairment of quality, for up to 14 d at 4 °C.