Intestinal digestibility of undegraded feed protein is an important parameter in modern protein evaluation systems for ruminants. Different methods may estimate the intestinal digestibility of a wide range of feeds. Three methods based on the: acid detergent insoluble nitrogen (ADIN) content, protein disappearance from the mobile bag and in vitro intestinal digestibility using a preincubation stage in both sheep and goats were compared. Different feedstuffs were used in the experiment. N post-ruminal availability of olive by-products was low and varied depending on factors such as year or treatment. Compared with the mobile bag technique, total N digestibility was overestimated by the in vitro method. If N disappearance from the mobile bag is accepted as a physiologically reasonable estimation for in vivo true protein digestibility, acid detergent soluble nitrogen (ADSN) content and the in vitro method may give good estimates of digestibility of N although some differences between by-products and conventional feeds are observed.