Conjugated linoleic acid (CLA) was produced at 300 mg l(-1) after 24 h culture of Lactobacillus reuteri in de Man-Rogosa-Sharpe medium containing 0.9 g linoleic acid (LA) l(-1) and 1.67% (v/v) Tween 80. CLA was mainly located in the extracellular space of the cells. Washed cells previously grown on LA were less active than unadapted washed cells in converting LA into CLA. Most of the CLA transformed by washed L. reuteri cells was located in cells or associated with cells. CLA production by washed L. reuteri cells was most efficient in conversion with 0.45 g LA l(-1) at pH 9.5 and 37 degrees C for 1 h.