The post-weaning growth check causes considerable economic losses in pig production. Some of the problems in susceptibility to disease have been associated with immunoregulation. For example, immaturity of the neonatal immune system, stress-associated and pathogen-induced immunosuppression have all been linked to increased disease susceptibility throughout the early post-weaning period. Studies in both infants and animal models suggest that dietary nucleotides have significant effects on the immune and gastrointestinal systems (see Carver, 1999). It has been suggested that under conditions of limited nucleotide intake, rapid growth or certain disease challenges, dietary (or preformed) nucleotides may spare the cost of de novo nucleotide synthesis and optimise the metabolic function of rapidly dividing tissues such as those of the gastrointestinal and immune systems. The aims of the current study were to determine the effects of a yeast-based nucleotide source (Ascogen™; Chemoforma Ltd, Switzerland) on performance, gut physiology, microflora and immunological parameters in post-weaned piglets.