Conjugated linoleic acid (CLA) is a collective term for a class of positional and geometric conjugated dienoic isomers of linoleic acid (LA) and has anti-cancer activity in experimental animals. We have previously observed that an isomeric mixture of CLA and trans-10,cis-12 (t10c12) inhibited cell growth in a dose-dependent manner whereas LA and cis-9,trans-11 (c9t11) had no effect. The present study examined whether the CLA mixture and t10c12 induce apoptotic cell death. TSU-Prl cells were incubated for three days in serum-free medium in the absence or presence of individual fatty acids, and the DNA fragmentation assay was performed. Cells treated with the CLA mixture or t10c12 produced a distinct oligonucleosomal ladder with different sizes of DNA fragments, a typical characteristic of cells undergoing apoptosis. By contrast, LA and c9t11 had no effect. Western immunoblot analysis of total lysates revealed that t10c12 reduced anti-apoptotic, 26 kDa, Bcl-2 protein levels by 49$pm$8% compared with controls, whereas this CLA isomer did not alter pro-apoptotic,21 kDa, Bax protein levels. These results suggest that growth inhibitory effect of the t10c12 CLA isomer may, at least in part, be attributed to Increased apoptotic death in TSU-Prl cells.