Measurement of N disappearance during the incubation of a feed in polyester bags in the rumen is a widely used procedure for determining the ruminal protein degradability characteristics of the feed. These can then be used in conjunction with measurements of rumen outflow rate to calculate the expected extent of degradation in the rumen, which is a necessary prerequisite for diet formulation. Using the polyester bag procedure rates of feedstuff protein degradation have been found to be lower when cereal diets are given than when forage diets are given. Whilst this has been attributed to lower levels of cellulolytic activity in the rumen of cereal-fed animals it has also been suggested that it may be due to the physical nature of the rumen contents causing blockage of the bag pores. In the present study the effect of feeding either an all-forage or a high-cereal diet on polyester bag-derived and in vivo estimates of soyabean meal (SBM) protein degradability in the sheep rumen was examined.