Lactobacillus probiotics have consistently shown in vitro properties of key importance in the prevention of diarrhoea at weaning. However, these probiotics have shown variable results in pigs in vivo, and it is important to determine the reasons for this variability if the efficacy of the preparations is to be improved. Hillman et al. (1993) reported that there are significant oxygen levels along the piglet intestine. As Lactobacillus spp. are primarily anaerobic bacteria, and are isolated and examined in vitro under anaerobic conditions, it is possible that the presence of oxygen along the piglet intestine could be exerting a detrimental action on their probiotic effectiveness. This experiment was set up to examine the influence of oxygen on two porcine Lactobacillus spp which have been previously demonstrated to inhibit the growth of Escherichia coli K88 in vitro (Hillman and Fox, 1994).