Probiotics have been found to stop pathogenic bacteria from colonising and flourishing in the gut and thus could potentially prevent illness and even death (Kyriakis et al., 1999). The organisms contained within probiotics are also suggested to be able to break down food into a higher number of volatile fatty acids, which increases energy production for the horse (Yuyama et al., 2004). Probiotics are however not very heavily researched in horses and the research which is present is either focused on neonates, young foals or older horses (Kumagai et al. 2004). Lactobacillus spp. are some of the most commonly used bacteria in equine probiotic supplements as they have shown good success in human based trials and seem to induce positive effects which would be useful to the horse (Laake et al., 2005). Research is however required in this area to investigate if Lactobacillus spp. can survive the acidic and bile environments of the equine gastrointestinal tract to a sufficient level to induce beneficial effects. The aims of this study were to discover if feeding a mixed culture probiotic supplement would alter the Lactobacillus spp. count in horses and whether this effect is similar in horses of different ages.