Aims : To assess strains of Lactobacillus , Lactococcus , Pediococcus and Bifidobacterium for their ability to produce the health-promoting fatty acid conjugated linoleic acid (CLA) from free linoleic acid. Methods and Results : In this study, strains of Lactobacillus , Lactococcus , Pediococcus and Bifidobacterium were grown in medium containing free linoleic acid. Growth of the bacteria in linoleic acid and conversion of the linoleic acid to CLA was assessed. Of the bacteria assessed, nine strains of Bifidobacterium produced the c 9, t 11 CLA isomer from free linoleic acid. The t 9, t 11 CLA isomer was also produced by some strains, but at much lower concentrations. Conclusions : The production of CLA by bifidobacteria exhibited considerable interspecies variation. Bifidobacterium breve and B. dentium were the most efficient CLA producers among the range of strains tested, with B. breve converting up to 65% linoleic acid to c 9, t 11 CLA when grown in 0.55 mg ml(-1) linoleic acid. Strains also varied considerably with respect to their sensitivity to linoleic acid. Significance and Impact of the Study : The production of CLA by probiotic bifidobacteria offers a possible mechanism for some health-enhancing properties of bifidobacteria and provides novel opportunities for the development of functional foods.