Growth performance was better among treated cattle as demonstrated by a significantly greater ADG of 130 g. The improved increase is probably due to the digestive process optimization induced by the treatment. The mixture of essential oils administered has been shown to improve ruminal fermentation activity and reduce methanogenesis, thereby reducing energy waste, reducing ammonia concentrations, and increasing microbial protein production as demonstrated by in vitro studies (Cardozo et al. 2004, 2005; Busquet et al. 2005, 2006). The effect of essential oils could be particularly interesting in intensive beef cattle rearing, because the antimicrobial effect is directly proportional to the ruminal pH drop from 6.5 to 5.5 (Busquet et al. 2005, 2006; Calsamiglia et al. 2007), a range that characterizes the fattening cattle diets. Concerning the immune response, essential oil administration has helped cattle to quickly recover to physiological conditions after the stressful events typically present in newly received beef calves. Bactericidal serum activity at day 25 was higher than the threshold of 90 %, and an improvement in immune response to vaccination was also observed. These positive effects can be attributed both to the rumen fermentation optimization and to the increase of dry matter intake (Yang et al. 2010a, b), but also to a direct effect of the essential oils with immune system cells. This second mechanism needs to be investigated further. The use of essential oils of cinnamaldehyde, eugenol, and capsicum in newly received beef cattle diet seems to be a successful strategy to improve growth performance and immune response during the adaptation phase.