Protein disappearance from the mobile nylon bag and acid detergent solubility of protein (ADSN) were estimated in different concentrate feedstuffs (guar meal, rapeseed meal, cottonseed meal, sunflower meal, dried sugar beet pulp, corn distillers dried grain, maize feed meal, maize gluten feed, pig hair meal, feather meal, hair-feather meal). Estimates with the mobile bag method included a 16 h preincubation in the rumen. Protein degradability varied both within and between feeds and degradabilities of feeds of plant origin were higher than for feeds of animal origin. The N content was generally higher in feeds of animal origin compared to feeds of plant origin, which makes animal origin feeds a good source of undegraded dietary protein. For corn distillers dried grain, disappearance obtained with the mobile bag was higher than acid detergent solubilities, whereas the opposite was the case for all other feeds tested, and most pronounced for feeds of animal origin and for dried sugar beet pulp. Generally, there was a low correlation between results obtained with the mobile bag and with acid detergent. If it is accepted that protein disappearance from the mobile bag is a good estimate for in vivo true digestibility, our results indicate that ADSN is not a satisfactory method for estimation of protein digestibility.