The digestion of heat-damaged protein.

I. The apparent digestibilities for chicks, operated on so as to allow separate collection of urine and faeces, of the nitrogen in a heat-damaged cod flour (C35) and of a control, freezedried cod muscle (Cz3) were 77 and 90 yo respectively. 2. The differences are similar to those found for rats in earlier work and considerably smaller than the differences found in nutritional value of the materials as sources of either lysine or methionine for chicks. 3. Chicks killed 3 h after a test meal containing C23 showed little more N in their small intestine than did those on a N-free diet; other chicks receiving C35 showed much more N remaining in the gut. 4. It is hypothesized that significant quantities of heat-damaged protein may remain undigested in the small intestine, but may then be de-aminated by fermentation in the caecum so that values for the digestibility of N and of individual amino acids may be misleadingly high. 5. This hypothesis is supported by the finding that in caecectomized chicks the apparent digestibility of the N of C35 was only 68 %, whereas the digestibility of Cz3 remained the same as in intact chicks.