Conjugated linoleic acid (CLA) is a mixture of positional and geometric isomers of linoleic acid containing conjugated unsaturation. CLA has been shown to inhibit initiation of mouse skin carcinogenesis by 7,12-dimethyl-benz[a]anthracene (DMBA), mouse forestomach neoplasia induced by benzo[a]pyrene and rat mammary tumourgenesis induced by DMBA. Furthermore, CLA was shown to be cytotoxic to human malignant melanoma, colorectal and breast cancer cell lines in vitro. Cis-9, trans-11-octadecadienoic acid was the only CLA isomer incorporated into phospholipids of mouse forestomach and rat mammary tumours and is considered to be the biologically active factor. The mechanisms whereby CLA exerts its anticarcinogenic activity is still uncertain. However, it has been found to modulate the activity of cytochrome P450 and reduce the induction of ornithine decarboxylase and protein kinase C, known tumour production indicators, and possibly inhibit protein and nucleotide biosynthesis. Some of the anti-carcinogenic properties of CLA may be due to its effective antioxidant properties. Dairy products are the richest dietary sources of CLA. In general, fat from meat of ruminant animals contains more CLA than fat from non-ruminants. Vegetable oils and margarines contain only small amounts of CLA. CLA is present in the tissue and body fluids of humans and blood serum levels can be increased by diets rich in CLA.