Grilled ground beef contains factors that inhibit the initiation of mouse epidermal carcinogenesis by 7,12-dimethylbenz(a)anthracene. Previously we isolated an active principal and characterized it as an isomeric mixture of conjugated dienoic derivatives of linoleic acid (CLA). We now show that synthetic CLA inhibits the initiation of mouse forestomach tumorigenesis by benzo(a)pyrene. Four and 2 days prior to p.o. treatment with benzo(a)pyrene, female ICR mice were given (a) CLA in olive oil, (b) linoleic acid in olive oil, or (c) olive oil alone or plus 0.85% saline (control groups). Three days later the cycle was repeated for a total of 4 times. At 30 wk of age, the mice were sacrificed. In three independent experiments, mice treated with CLA developed only about half as many neoplasms/animal as mice in the control groups (P less than 0.025); in two of the experiments tumor incidence was also reduced (P less than 0.05). There were no significant differences in food intake or body weight among the groups. High-performance liquid chromatography/gas chromatography analysis established that, following intubation, only the c-9, t-11 CLA isomer was incorporated into forestomach phospholipids. In studies aimed at elucidating the mechanism of action, we found that CLA is an effective antioxidant. Under the conditions of the test CLA was more potent than alpha-tocopherol and almost as effective as butylated hydroxytoluene. These observations indicate that CLA might serve as an in situ defense mechanism against membrane attack by free radicals and may, at least in part, explain the anticarcinogenic properties of CLA.