Degradability in the rumen of several protein sources was determined by suspending from 12 to 13 g of feedstuff in dacron bags into the rumen for 0, 2, 6, 12, 24, 36, 48, and 72 h. Rumen cannulated lactating Holstein cows consuming a diet of corn silage, alfalfa, soybean, and high moisture corn were used. Degradability of protein varied from 18.6% for corn gluten meal to 72.3% for soybean meal. Gel electrophoresis was used to monitor rates of degradation in the rumen of fractions of corn gluten (CGM), CORN, cottonseed (CSM), peanut (PM), and soybean meal (SBM) protein fractions. Fractional degradation rates in the rumen were determined from densitometric analysis of stained polypeptides bands on SDS-PAGE gels. Acidic subunits of soybean glycinin were degraded at a faster rate than basic subunits (.144 vs .104 h⁻¹). Rates of degradation of zein in corn and corn gluten meal were .026 and .015 h⁻¹, respectively. Protein degradability estimated by using B subfractional components did not differ from degradability measured using total B fractions. Lag phase associated with dacron bags suspension technique did not change effective degradability. Protein solubility in SDS-PAGE sample buffer was highly correlated (R²=.958) with in situ protein degradability of CORN, CSM, DBG, FM, PM, and SBM. Different rates of degradation of each fraction may directly influence protein and amino acid contribution to the animal. / Master of Science