This research determined the extent of ruminal degradation of a transgenic protein (Cry1Ab) expressed in kernels of maize (Zea mays) cultivar DKC53-32. Cracked kernels of transgenic maize (Bt+) were subjected to rumen digestion using in situ experiments with two ruminally cannulated Holstein cows. A non-transgenic maize cultivar (Bt-) was used as a control for crude protein (CP) degradation. CP and Cry1A concentrations from dacron polyester bags inserted in the rumen were determined for 48 h. The Bt- cultivar had greater CP content (P<0.05) than the Bt+ maize, but no difference (P>0.05) was found in ruminal degradable protein and the CP rate of degradation. The Cry1Ab protein was detected throughout ruminal digestion. The amount of Cry1Ab protein remaining after 16 and 24 h of ruminal degradation was 85 and 60%, respectively. These results indicated that a significant fraction of Cry1Ab protein was not digested in the rumen. This is the first report of the rumen degradation rate of a transgenic protein.