Ovine ruminal fluids were incubated in vitro at 39 degrees C under CO2 either alone or with a combination of linoleic acid (LA) (1.67 g/l), a low (0.167 mu g/ml) or high (1.67 mu g/ml) level of selenite (Se(IV)), or selenate (Se(VI)). The high level of Se(IV) resulted in a decrease of CLA isomer accumulation in in vitro incubated cultures with LA. Low and high levels of Se(VI) in ruminal fluids elevated the content of CLA isomers and trans-vaccenic acid in fluids incubated with LA.