When proteins are severely heated, in the presence or absence of sugars, the fall in nutritional value appears to be largely, although not completely, explained by reduced protein digestibility. All amino acids are considerably reduced in availability; which is in contrast to early Maillard damage which occurs under mild conditions of heating or storage in the presence of reducing sugars and affects mainly lysine. It seems that corss-linkages are formed in the severely heated protein or protein-sugar mixes and that these reduce the rate of protein digestion. When freeze-dried chicken muscle (15% H2O content) was severely heated its protein shown a great fall in digestibility but its value for supporting growth in the rat had fallen even more. Aspartyl-lysine and glutamyl-lysine cross-linkages, which had been formed during heating, appeared to be as digestible as the total N component and it seems that once they are released from the protein chain they can be absorbed and utilized.