Lactulose is recognized world-wide as one of the most clinically reliable bifidogenic factor. Lactulose is used in various types of food products (infant formula, baby food, confectionary, soft drink, milk products) and also pharmaceutically to improve hepatic encephalopathy and constipation (Mizota, 1996; Strohmaier, 1998). The object of this study was to investigate the influence of concentration of lactulose on growing Bifidobacterium lactis in milk. A bifidogenic lactor, lactulose was added (1, 2, 3, 4 and 5%) into 100g of milk. The milk samples were inoculated with Bifidobacterium lactis (BB-12, Chr. Hansen, Denmark) and incubated at 38 deg C for 16 hours. Lactulose as a prebiotic influences the growth of bifidobacteria in milk. Trends of development of acidity in the milk samples with or with out lactulose were not similar. Laboratory studies have generally shown that growth of Bifidobacterium lactis in milk depends on the concentration of lactulose. Data on the final cell count in fermented milk indicates that increasing the lactulose concentration from 2% to 3% enhanced the growth of the Bifidobacterium lactis, whereas no significant difference between lactulose concentration 4-5% and control sample was observed. Present result furthermore indicate that finding combination of prebiotic and probiotic pairs where the prebiotic would benefit the specific probiotic strain, e.g. during production and formulation into foods, is not a simple task.