1. To assess the part played by the microflora in the digestion of proteins, the amino acid composition of the excreta and the apparent and true digestibilities of individual amino acids were measured in germ-free (GF) and conventional (CV) chicks.

2. Three diets were used: diet 1, nitrogen-free; diet 2,280 g protein/kg as (g/kg): casein 80, gelatin 100 and freeze-dried egg albumen 100 and diet 3,280 g protein/kg as diet 2 but with heat-damaged instead of freeze-dried egg albumen. Half of the GF and half of the CV chicks received the N-free diet in the first 7 d of test and the other half of each group received either diet 2 or diet 3. In the second 7 d test period those chicks which had been given the N-free diet received either diet 2 or diet 3 while those which had been given protein diets received the N-free diet. Total amino acids were measured in hydrolysates of the soluble and insoluble fractions of the excreta collected in the last 3 d of each test period.

3. The amino acid composition of the soluble fraction of the excreta of chicks given either diet 2 or diet 3 differed markedly from that of chicks given the N-free diet. The amino acid composition of the insoluble fraction of the excreta of chicks given diet 2 was similar to that of chicks given the N-free diet, whereas that of chicks given diet 3 was markedly different and resembled egg albumen in composition.

4. In the soluble fraction of excreta from CV chicks given diets 1 and 2 the proportions of threonine, serine and glucosamine were lower and those of methionine, leucine, isoleucine and phenylalanine were higher than in those from GF chicks, particularly on diet 1. In the insoluble fraction of excreta from CV chicks given these two diets, compared with GF chicks, there were lower proportions of serine and proline and higher proportions of cysteic acid and lysine, the latter particularly with diet 1.

5. Lower proportions of threonine, serine and glucosamine were also observed in the soluble fractions of excreta from the CV chicks given diets 1 and 3, compared with GF chicks, whereas the proportion of glutamic acid was higher. With these two diets the insoluble fraction from CV chicks contained a higher proportion of alanine. In each instance the environmental effect was greater with diet 1.

6. No effect of environment on either apparent or true digestibility of individual dietary protein amino acids was demonstrated, with the exception of threonine in diet 2, the true digestibility of which was higher in GF than in CV chicks.

7. It was concluded that the gut microflora of the chick had little influence on the digestion of the proteins in the diets tested, but may serve an important role in the degradation of endogenous proteins and the recycling of N.