This study was performed to evaluate the isomer-specific cytotoxic effects of conjugated linoleic acid (CLA) on rat hepatoma dRLh-84 cells in vitro. A 10trans,12cis (10t,12c)-CLA showed a strong cytotoxic effect on dRLh-84 cells in culture, whereas no such effect was observed with 9cis,11trans (9c,11t)-CLA or linoleic acid. The optimum concentration for induction of cytotoxicity by 10t,12c-CLA was 5 to 10 microM, but the effect was alleviated at higher concentrations. Coincubation with oleic or palmitoleic acid and 10t,12c-CLA cancelled the cytotoxic effect, but other major saturated or polyunsaturated fatty acids and eraidic acid did not interfere with 10t,12c-CLA-induced cytotoxity. The cytotoxic effect was also alleviated by alpha-tocopherol (alpha-toc) and alpha-tocotrienol but not by any other antioxidant reagent examined. Significant cytotoxicity of 10t,12c-CLA was detected after only a 15-min incubation, and the most noticeable effect was seen after 3 h. After incubation with 10t,12c-CLA at 10 microM, an additional 90 microM of 10t,12c-CLA or 100 microM of alpha-toc was also able to alleviate the cytotoxicity. When cells were treated with 10 microM 10t,12c-CLA for more than 48 h, treatment with additional CLA or alpha-toc could not prevent cell death.