A dual flow continuous culture system was used to determine the effects of four protein sources (soybean meal, Ca-lignosulfonate treated-soybean meal, blood meal, and feather meal), supplied individually or in combination in diets composed predominantly of alkaline hydrogen peroxide-treated wheat straw, on ruminal microbial fermentation and amino acid flow. Diets containing blood meal had lower organic matter and fiber digestion, NH3 N and VFA concentrations, and CP degradation but higher non-NH3 N, dietary N, total amino acid, and essential amino acid flows. Feather meal fed alone or combined with other sources was not different from the blood meal diet in organic matter and fiber digestion. Combining treated soybean meal and blood meal resulted in similar organic matter and fiber digestibilities compared with the soybean meal diets. This combination was similar to the treated soybean meal diet in VFA concentration, non-NH3 N, and essential and total amino acid flows; however, amino acid profile was different with this combination, showing higher histidine and leucine flows. Results from this study suggest that amino acid profiles of digesta leaving the rume may be manipulated by choice of protein supplement when diets containing a low protein, highly fermentable feedstuff such as alkaline hydrogen peroxide-treated wheat straw are fed.