The usefulness of acid detergent insoluble nitrogen (ADIN) to predict digestibility of rumen undegradable protein (UDP-D) of heat processed and unprocessed plant proteins was investigated. A series of drum roasted protein sources, sunflower, cottonseed and soybean oilcake meals, in addition to whole soybeans, whole sunflowerseed, whole cottonseed and whole lupinseed, were evaluated in this study to test the relationship between ADIN (% of total N) and UDP-D. Using ADIN as a screening parameter, five samples on average of each protein source together with the unprocessed control were selected for an in situ study using the mobile bag technique (MBT) to determine UDP-D. Digestibility of undegradable protein was not significantly decreased as a result of heat treatment, when ADIN values were below 12-15%, but progressively decreased when ADIN was higher, suggesting substantial heat damage. Ruminal nitrogen digestibility also decreased (p <0.05) with heat treatment. Digestibility of undegradable protein was predicted with a = 0.72 and error of estimate (Sy.x) of 4.78%. The prediction equation (p <0.001) of UDP-D (%) = 91.9 â€“ 0.025 (ADIN, % in DM)2, could therefore be a satisfactory guideline for predicting UDP-D and heat damage of the above-mentioned heat proÂ¬cessed and unprocessed plant protein sources.