Dietary supplements of a mixture of CLA isomers have been shown to reduce body fat accretion in mice and pigs with the trans-10, cis-12 isomer probably being responsible for the anti-lipogenic effects. The efficacy of calcium salts of a mixture of CLA methyl esters containing trans-10, cis-12, on milk fat synthesis, is relatively low (Bernal-Santos et al., 2003) and reported lack of effects on growth may be due to low efficacy and the short duration of CLA supplementation, during the last 32 or 60 days before slaughter, and the relatively low level of trans-10, cis-12 CLA fed. Shingfield et al (2004) demonstrated the potential of rumen protected supplements of a mixture of CLA isomers to reduce milk fat content and improve the energy status of dairy cows during early lactation. The current experiment used a lipid-encapsulated supplement containing equal amounts of cis-9, trans-11 and trans-10, cis-12 CLA methyl esters to study the effects on the performance and carcass characteristics of Limousin steers during a 100d finishing period.