Four protein sources were incubated in situ to estimate AA disappearance. Bags containing either soybean meal, corn gluten meal, herring meal, or meat meal were washed in water or suspended in the rumen of two Holstein cows for 8, 12, 16, 24, 48, 72, and 120 h. Cytosine, a bacterial marker for microbial contamination, was used to correct the essential AA profile for microbial contribution to determine the residual essential AA composition of the protein sources after incubation. Ruminal disappearance of individual essential AA was different among feedstuffs. Relative to original feed protein, soybean meal and corn gluten meal decreased the concentration of specific essential AA in the RUP. Concentration of all essential AA, except Arg and His, increased in undegraded meat meal protein. The difference between original and residual AA concentrations in herring meal approached statistical significance. Use of the original AA profile of the feed protein to predict essential AA available for absorption is not accurate because accuracy differs with sources.