Dietary protein degraded to various extents by varying the time of rumen incubation was prepared from eight concentrates and four roughages. Intestinal digestibility was obtained using the mobile bag technique on intact protein and on the samples of undegraded dietary protein from each feed. The results showed that increased degradability of the protein in the rumen decreased the intestinal digestibility, which shows that feedstuffs contain a protein fraction which is both undegradable in the rumen and indigestible in the intestine. The results thus indicate that it is possible to calculate the intestinal digestibility from information on the intestinal digestibility of the protein in the intact feed at any degradability estimated. The results clearly show that intestinal digestibility of undegraded dietary protein cannot be considered as a constant value as used in most protein evaluation systems introduced in recent years.