1. Seven isonitrogenous diets were prepared containing soya-bean meal and dried grass either unsupplemented, or supplemented with wheat starch or paper or equal mixtures of both.
2. The diets were allocated according to a balanced incomplete block design to seven Clun Forest wether sheep, each fitted with a re-entrant duodenal cannula.
3. After each sheep had received a diet for 6 d, daily samples of digesta were collected automatically for the next 3 d.
4. The amounts of dry matter, gross energy and major nitrogenous components consumed in the diet, passing into the duodenum and excreted in the urine and faeces were determined.
5. A novel method was applied to estimate the proportion of the total amino acids passing into the duodenum which was of microbial origin.
6. The amounts of microbial total amino acids synthesized were compared with the amounts of energy disappearing in the rumen for each of the seven diets tested. For the starch- and paper-containing diets an average of 14.7 g microbial total amino acid passed into the duodenum per MJ energy disappearing in the rumen, whilst for the diets containing either paper or starch alone the mean value, 6.1 g/MJ, was significantly lower (P < 0.05).
7. The results of the experiment suggested that the energy released from a mixture of starch and paper was utilized more efficiently for microbial protein synthesis than when the energy was provided by supplements of either paper or starch alone.