PURPOSE: To promote the proliferation of bifidus bacteria, by adding LMO to a proliferation medium and to obtain a fermented dairy product containing active bifidus bacteria in high concentration by adding additional MO to the produced dairy product.

CONSTITUTION: The number of cells of bifidus bacteria in a final dairy product (e.g. yoghurt) can be increased by adding 0.01W25 units of LMO (lactic acid monooxygenase) to 1ml of sterilized and cooled compounded mixture before or during the addition of a starter or during the filling of the product in a container. For the purpose of preventing the decrease in living cell number during the distribution of the final product from the factory to the consumer, LMO is added to the liquid cultured in a tank, cooled and crushed to curd in an amount to obtain a final product (lactic beverage containing bifidus bacteria) containing 0.01W20 units of LMO per 1ml of the drink. The reaction catalyzed with LMO produces acetic acid, carbon dioxide and water, all of which are non-toxic.