Conjugated hnoleic acid (CLA), a group of polyunsaturated fatty acids occurring naturally in dairy products but also produced by certain strains of human intestinal bifidobacteria is known to exhibit potent anticancer effect both in vivo and in a range of tumour epithelial cell lines The HT-29 human colon cancer cell line was used in this study as an in vitro model to investigate the effects of CLA and ¿raws-vaccemc acid (¿- VA), a putative precursor of c9, t \ \ CLA on markers of growth, differentiation and apoptosis Sodium butyrate, which maintains a balance between cell proliferation, differentiation and apoptosis in intestinal epithelium was used as positive control. For comparative purposes, parallel experiments were performed with hnoleic acid HT-29 cells were sensitive to the growth inhibitory effects of a CLA mixture of isomers and to three of its constituent isomers, c9, ¿11 CLA, ¿10, c l2 CLA and t9, t \ \ CLA at physiological levels /-VA was cytotoxic to the HT-29 cells at concentrations greater than 70jjM and was less inhibitory than CLA treatments. The CLA mixture of isomers, c9, t\ 1 CLA and ¿10, c l2 CLA showed evidence of apoptosis of HT-29 cells as reflected by annexin binding, measured by flow cytometry All CLA isomers induced carcinoembryomc antigen (CEA) and showed varying levels of reduction in histone deacetylase (HDAC) activity. Increased level of ceramide was observed when cells were incubated with the CLA mixture of isomers. In this study the gas chromatographic methods for analysis of CLA and ¿-VA m HT-29 cancer cells was validated This study provided evidence for cellular bioconversion of ¿-VA to c9, ¿11 CLA in HT-29 cells CLA isomers altered fatty acid composition in HT-29 cells which may be via modulation of fatty acid synthase (FAS) and stearoyl-CoA desaturase (SCD) activities. This study indicated that the antiproliferative effect of CLA on HT-29 colon cancer cell line may be mediated by differentiation and apoptosis and by modulation of FAS and SCD activities.