Lactic acid bacteria have an inefficient proteolytic system. Therefore, cultivation media which may have high protein content are usually supplemented with yeast extract or protein lysates (peptones). These additives might be conveniently replaced by in situ treatment of the cultivation medium with proteolytic enzymes or proteolytic microbes. Lactobacillus salivarius ssp. salicinius, a lactic acid bacterium species that can grow at high salt concentration, was used to ferment lactic acid in cheese whey (with 3 gl(-1) whey protein content) and lactose mother liquor (90 gl(-1) lactose, 9 gl(-1) proteins, 30 gl(-1) minerals). The contribution of protease enzymes or proteolytic microbes to acid production by lactobacilli was examined. Efficient conversion of lactose to lactic acid was obtained in the presence of additional proteolytic activity. Fastest acid production was obtained with the addition of protease enzymes. However, almost equally efficient acid production was obtained by treating the medium with Bacillus megaterium. The results show that fast and complete conversion of lactose to lactic acid can be obtained in dairy by-products without expensive additives.