Initial rates of in vitro degradation of alfalfa proteins were studied. Fresh, 24-h wilted, and ensiled forages were homogenized before analysis for total proteins. Some of the homogenates were fractionated by differential solubility in 10 and 40% ammonium sulfate, followed by ultrafiltration of the 40% salt-saturated solution. The protein fractions obtained were chloroplast membrane proteins (fraction 3), soluble proteins from plant cell cytoplasm and the chloroplast (fraction 2), and proteins remaining soluble in the extracted 40% salt-saturated solution (fraction 2B), respectively. Total and fraction 3 silage proteins were degraded faster than the respective fresh and wilted proteins. There were no treatment effects on the rates of degradation of the soluble proteins of fractions 2 and 2B. Protein fractions from fresh and 24-h wilted alfalfa degraded, from greatest to least, in the following order: fractions 2, 2B, and 3. Degradation rates for fractions 2 and 2B of ensiled forages were similar but greater than that of fraction 3. Alfalfa proteins were degraded rapidly in the rumen, and soluble proteins were degraded faster than the chloroplast membrane proteins. Ensiling of alfalfa increased the rate of degradation of chloroplast membrane proteins, but neither wilting nor ensiling affected degradation rates of the soluble protein fractions 2 and 2B.