Augmenting the supply of specific amino acids to pigs and poultry is straightforward cbinpared with ruminants where fermentation in the forestomach may reduce the proportion of the supplement which ultimately reaches the small intestine. Recently there have been several attempts to develop amino acid supplements which escape rumen degradation, thus optimising the increase in duodenal amino acid supply. The work reported in this study provides an in vivo assessment of one such product - namely lysine and methionine coated with a pH sensitive polymer.