The protein degradability of various South African ruminant feed protein sources was determined in the rumen of cannulated lactating dairy cattle. The effective protein degradability was calculated at different fractional outflow rates, using the nylon bag technique. The degradability values ranged from 96,2; 91,9 and 88,7% for whole sunflower seeds to 32,2; 25,1 and 23,0% for blood meal at fractional outflow rates of 0,02; 0,05 and 0,08/h, indicating a large variation in the resistance of protein sources to ruminal breakdown. It is proposed that when protein sources are bought they should be evaluated on their rumen undegradable protein (UDP) content rather than crude protein content, as this would reflect the true nutritional value of a protein source more accurately.