Protein degradability was estimated in adult cattle with well-developed rumen (oxes, dairy cows). All experimental animals were endued with ruminal cannulas (10 cm). The feeding ration consisted of lucerne hay (4 kg), maize silage (6 kg), concentrate (2kg) and mineral-vitamin supplement. Ground (mesh size 2 mm) roughage in the amount of 1.3 g was weighed intu bag with the accurary of 10-4 g. Ground (mesh size 3 mm) concentrate was weighed into bags in the amount of 1.3 g. In vivo incubation intervals were 0, 2, 8, 16, 24 and 48 hours (for roughage also 72 h). Bags were placed into the rumen using a carrier. The effective degradability (deg) was calculated according to the equation (Orskov and McDonald, 1979). The equation: deg = a + b (1-e-ct) was just, where a = watersoluble feed protein fraction; b = insoluble, potentially degradable fraction; c = speed of degradation of fraction b; and k = rate of passage of particles from rumen. The value of k was 6% (INRA, 1988). Correction for a