The prodn. of lactic acid in a cell-recycle reactor was investigated. Studies were conducted with L. helveticus on a supplemented whey permeate medium. At 42 Deg and pH 5.5, the biomass was kept const. at 21.3 g/L. The volumetric lactic acid productivity was approx. doubled from 7.8 to 15.8 g/L-h by increasing the diln. rate from 0.18 to 1.05/h. At the same time, the L(+)-lactate fraction increased from approx. 60% to 70%. Using this recycled system, the volumetric lactic acid productivity was about twice as much as with a nonrecycled system. At a temp. of 45 Deg, the performance of the system was significantly lower than at 42 Deg. Replacing 60% of the yeast ext. by skim milk decreased productivity by only 25%. [on SciFinder (R)]