Using a co-cultivation system developed previously, positive interaction for cell growth between Bifidobacterium adolescent-is and Propionibacterium freudenreichii was evaluated. The total dry cell weight (DCW) of these two strains obtained in the co-cultivation system was 1.5-1.7-fold of the sum of the DCWs obtained in two single cultivations of each bacterium. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.