Some in-feed antibiotic growth promoters have been suspended from use within the EU. Alternatives to these antibiotics are actively being sought, especially ‘natural’ alternatives, such as essential oils, to try and maintain the performance advantage attributed to the use of these antibiotics. Some essential oils, e.g. thyme and origanum, have been shown to have anti-microbial activities (Hammer et al., 1999). The active compounds responsible for this property have been identified, and include cinnamaldehyde, cineol and eugenol. A specific formulation of essential oils reinforced with their active compounds has been combined into a form suitable for use as a feed additive (Multi-Functional Feed Additive, MFA). An experiment was conducted to determine the effect of this MFA on the food conversion ratio (FCR) of calves.