The effects of temperature, pH, and medium composition on lactic acid production by Lactobacillus casei were investigated. The highest lactic acid productivity values were obtained at 37degreesC and pH 5.5. The productivity was 1.87 g dm(-3) h(-1) at 37degreesC in shake flasks. In the fermenter, a productivity of 3.97 g dm(-3) h(-1) was obtained at pH 5.5. The most appropriate yeast extract concentration was 5.0 g dm(-3). Whey yielded a higher productivity value than the analytical lactose and glucose. Initial whey lactose concentration did not affect lactic acid productivity. MnSO4.H2O was necessary for lactic acid production by L casei from whey. Product yields were approximately 0.93 g lactic acid g lactose(-1). (C) 2004 Society of Chemical Industry.