Conjugated linoleic acids (CLA) are a group of positional and geometric isomers of linoleic acid produced by ruminant animals. Despite animal studies showing that CLA is an effective agent in preventing mammary and colon tumour development there is a need to determine isomeric-specific effects of CLA and identify molecular targets in tumour tissues. Mammary (MCF-7) and colon (SW480) tumour cell lines were used in this study as in vitro models to investigate effects of CLA on growth and membrane-initiated signalling pathways. Both cell lines were sensitive to the growth-inhibitory effects of a CLA mixture o f isomers and to the individual /10, cl2- and c9, fll-CLA isomers at physiological levels. CLA isomers modulated arachidonic acid distribution among cellular lipids and altered the prostaglandin profile of both cell lines suggesting interference in an eicosanoid signaling pathway. Similar effects were observed in cells treated with CLA-cnrichcd milk fat obtained from cows fed on rapeseed supplemented pasture. This study also provided evidence for cellular bioconversion o f vaccenic acid to c9. i\ I CLA isomer. Western blot analysis of a panel o f apoptosis regulatory proteins (bcl-2, bax, Apaf-1 and caspase 3) in the SW480 cell line indicated that induction of apoptosis by CLA isomers contributed to growth inhibition in this cell line.