The linoleic acid isomerase enzyme from Propionibacterium acnes responsible for bioconversion of linoleic acid to trans-10, cis-12 conjugated linoleic acid (t10, c12 CLA) was cloned and overexpressed in Lactococcus lactis and Escherichia coli, resulting in between 30 and 50% conversion rates of the substrate linoleic acid to t10, c12 CLA. The anti-proliferative activities of the fatty acids produced following isomerization of linoleic acid by L. lactis and E coli were assessed using the human SW480 colon cancer cell line. Fatty acids generated from both L. lactis and E coli contained a mixture of linoleic acid and t10, c12 CLA at a ratio of similar to 1.35 : 1. Following 5 days of incubation of SW480 cells with 5-20 mu g ml(-1) (17.8-71.3 mu M) of the t10, cl 2 CLA, there was a significant (P<0.001) reduction in growth of the SW480 cancer cells compared with the linoleic acid control. Cell viability after treatment with the highest concentration (20 mu g ml(-1)) of the t10, c12 CLA was reduced to 7.9 % (L. lactis CLA) and 19.6 % (E coli CLA), compared with 95.4 % (control linoleic acid) and 31.7 % (pure t10, c12 CLA). In conclusion, this is believed to represent the first report in which recombinant strains are capable of producing CLA with an anti-proliferative potential.