A method of estimating the exogenous contribution to the duodenal amino acid supply in ruminants, based upon in vitro fermentation, is proposed. Analyses of five grass varieties, maize and maize/grass mixtures by the proposed method, reveals that in all cases histidine is the limiting amino acid for milk production. Comparison of the milk production potential predicted from the duodenal amino acid supply with that predicted from digestibility values, suggests that amino acids might be limiting for milk production in the case of certain feedstuffs.