Manipulation of amino acids (AA) supply to the duodenum is constrained in ruminants by insufficient knowledge on the AA profile of undegradable protein. There is evidence that rumen degradability and intestinal digestibility of individual AA may differ from that of nitrogen (N) or total AA (TAA) (Erasmus et al., 1994, O'Mara et al., 1997) although differences in intestinal digestibility are probably of relevance only in roughage and overheated feeds (Van Straalen et al., 1997). The objective of this experiment was to provide more information about the relative influence of rumen degradation and intestinal digestion on the AA profile of the absorbed fraction of four protein sources commonly used in beef cattle rationing.