1. An experiment was conducted with three sheep maintained entirely by intragastric nutrition to estimate the digestibility of isolated individual constituents and amino acids (AA) of rumen micro-organisms (RMO) in the small intestine.
2. Five levels of RMO were infused into the abomasum. The apparent and true disappearance of the individual components were measured by regression of abomasal input on the passage at the ileum.
3. The true digestibility values of N, AA-N, DNA and RNA were 0·82, 0·85, 0·81 and 0·87, respectively.
4. The digestibility of individual AA varied between 0·80 and 0·88, the only exceptions being diaminopimelic acid (0·37), histidine (0·68) and cystine (0·73), which were significantly lower than the average (0·847).
5. The endogenous components in the ileal fluid in sheep given protein-free infusions, expressed in mg/kg live weight0·75 per d, were total N 42 and AA-N 20.