Positive interactions between Bifidobacterium longum or Bifidobacterium breve and Propionibacterium freudenreichii that enhance cell growth were evaluated using a co-cultivation system. The total dry cell weight of B. longum and P. freudenreichii from co-cultivation was 10.6 g. This value is 1.2-fold higher than the sum (9.1 g) of the maximum dry cell weights obtained from the cultivation of each bacterium. Currently, the total dry cell weight of co-cultivated B. breve and P. freudenreichii was 21.3 g. The total dry cell weight of B. breve and P. freudenreichii is 1.6-fold higher than the sum (13.2 g) of the maximum dry cell weights from two single cultivations of each bacterium. These data are direct evidence for a cooperative interaction between the two microorganisms. Copyright © 2013 The Society of Chemical Engineers, Japan.