Conjugated linoleic acids (CLA), a mixture of positional and geometric isomers of octadecadienoic acids with conjugated double bonds, natural components of ruminant tissues and milk and have been demonstrated to have anti-cancer and metabolic activities-of potential benefit to human health. The effect of breed, Vitamin E and the feeding growing lambs fat supplements containing polyunsaturated fatty acids (PUFA) was investigated on the levels of CLA in lamb muscle and liver neutral lipids. A total of 96 male lambs were used in a 2 x 3 x 2 factorial design: 2 breeds, Suffolk x Lleyn and Scottish Blackface, 3 diet supplements, Megalac, linseed and linseed plus fish oil (a 50:50 mixture on an oil weight basis of fish oil and linseed, linfish) and two levels of Vitamin E (100 or 500 mg/kg feed). The lambs were fed isonitrogenous diets containing 50 g fat/kg DM ad libitum. At approximately half of the mature live weight for each breed, animals were slaughtered. Semi-membranosus muscle samples were removed 24 h post-mortem and lipids extracted with chloroform-methanol. Neutral lipids were isolated from the lipid extract using silicic acid chromatography. Lambs fed the Megalac supplement had significantly less muscle neutral lipid fatty acids than lambs fed linseed and linfish supplemented diets (Megalac 1859, linseed 2535 and linfish 2500 mg/100 g muscle, respectively). Muscle content of neutral lipid CLA (mg/100g muscle) was 12.3, 20.7 and 25.8 for the three feeds, respectively (S.E. 1.8, P < 0.01) and trans 18:1 was 39, 53 and 94 mg/100 g muscle (S.E. 7.0, P < 0.0 1) for Megalac, linseed and linfish diets, respectively. Liver content of neutral lipid CLA were 27.0, 24.4, and 20.9 mg/100 g liver (S.E. 2.9, NS) and trans 18:1 content were 5 8, 69, 97 mg/100 g liver (S.E. 10.0, P < 0.01) for Megalac, linseed and linfish fed lambs, respectively. The present study demonstrates that CLA concentrations in lamb meat can be enhanced by the addition of polyunsaturated fatty acids to the diets and further research is needed to characterise breed and Vitamin E effects on CLA content of lamb tissues. (C) 2003 Elsevier B.V. All rights reserved.