The influence of the microflora of the alimentary tract on protein digestion in the chick.

I. The course of digestion of freeze-dried and heat-damaged egg albumen labelled with 14C in all its constituent amino acids was compared in germ-free and conventional chicks. The albumen was given at a level of 10 % in a test meal containing 28 % protein. Samples of digesta were taken 5 h later and analysed for 14C and nitrogenous constituents. 2. After administration of unheated albumen, II yo of the '*C remained in the germ-free gut, compared with 23 % in the conventional gut. The poorer digestibility of the heat-damaged albumen was shown by recovery of 42 and 47 % of the 14C from the gut of germ-free and conventional birds respectively. 3. With both diets there was a higher 14C:nitrogen ratio in the contents of the lower gut of conventional birds. Most of the N remaining in the gut was composed of protein or the hydrolysis products of protein, the amounts of which tended to be higher in the germ-free environment; urea and uric acid formed only a small proportion of the total N. There was less urea and more uric acid in the lower gut and excreta of conventional chicks. 4. It was concluded that microbial activity did little to increase the availability of protein to the chick. However, its effect on the nitrogenous material in the lower part of the alimentary tract might result in low recovery of N in the faeces and thus lead to erroneously high estimates of protein quality in tests with conventional chicks.