To investigate the ability of selected probiotic bacterial strains to produce conjugated linoleic acid (CLA) and also to estimate the biohydrogenation kinetics of Lactobacillus acidophilus on the production of CLA from free linoleic acid (LA). Six probiotic bacteria, Lact. paracasei, Lact. rhamnosus GG, Lact. acidophilus ADH, and Bifidobacterium longum B6, Lact. brevis, and Lact. casei, were used to examine their ability to convert LA to CLA. LA tolerance was evaluated by addition of different LA concentrations in MRS broth. Lact. acidophilus showed the major tolerant to LA and the greatest CLA-producing ability (36-48 mu g ml(-1) of CLA). The rate-controlling steps were k(2) and k(1) for the addition of 1 and 3 mg ml(-1) of LA, respectively. The percentage of CLA conversion was higher in MRS broth supplemented with 1 mg ml(-1) (65%) than 3 mg ml(-1) (26%). The results provide useful information and new approach for understanding the biohydrogenation mechanisms of CLA production. This study would help elucidate the pathway from LA to stearic acid (SA), known as biohydrogenation. In addition, the use of selected probiotic bacteria might lead to a significant improvement in food safety.