It is desirable to increase the level of conjugated linoleic acid (CLA) in milk as a health benefit in human nutrition. CLA has been shown to affect carcinogenesis, atherosclerosis, diabetes, the immune system, bone mineralization, body fat accretion and nutrient partitioning. The predominant CLA isomer present in foods from ruminants is cis-9, trans-11 CLA. It is formed in the rumen by anaerobic bacteria as an intermediate in the hydrogenation of linoleic acid. Recent evidence has shown that CLA can also be produced in the mammary gland by desaturation of trans-11 C18:1. Previous researchers have used various oils or oil seeds to try and elevate CLA levels in milk. A problem with this approach is that most oils contain mixtures of fatty acids so responses cannot be attributed to individual acids. Up to now there has been no report looking at how individual fatty acids affect CLA production. The objective of this work was to separate the effects of linoleic and linolenic acids on CLA production in dairy cows.