The production of lactic acid from deproteinized whey by mixed cultures of free and coimmobilized Lactobacillus casei and Lactococcus lactis cells in batch and fedbatch culture was investigated. Fedbatch culture proved to be a better fermentation system for the production of lactic acid than batch culture. The maximum lactic acid concentration (46 g l(-1)) in fedbatch culture was obtained with both free cells mixture and coimmobilized cells at a substrate concentration of 100 g l(-1) and a feeding rate of 250 mi h(-1). In repeated fedbatch culture, coimmobilized L. casei and L. lactis cells gave a higher overall lactic acid concentration compared with the free cells mixture. The coimmobilized L. casei and L. lactis cells in Ca-alginate beads retained their ability to produce lactic acid for 20 days.