Holstein cows in early lactation were used to compare three combinations of extruded whole soybeans and corn gluten meal to soybean meal. Treatments were (as a percentage of supplemental crude protein) 100% soybean meal; 75% whole soybeans extruded at 149°C, 25% corn gluten meal; 50% extruded soybeans, 50% corn gluten meal; and 25% extruded soybeans, 75% corn gluten meal. Diets were formulated to be 37.5% corn silage, 12.5% alfalfa cubes, and 50% concentrate (dry matter). Dietary crude protein was 15.7% of dry matter, and supplemental protein sources supplied 27% of total dietary protein. Diets were fed as total mixed rations and data were collected from 4 to 116 d postpartum. Milk and fat-corrected milk yields ranged from 31.0 to 34.3 and from 29.3 to 33.3 kg/d, respectively, and were greater for cows fed soybean meal than those cows fed 75 or 50% of their supplemental protein as extruded soybeans. Milk yield of cows consuming 75% of their supplemental protein from corn gluten meal was similar to all other treatments. Ruminal ammonia concentrations were lower in cows fed extruded soybeans and corn gluten meal compared with those fed soybean meal and ranged from 9.1 to 12.4mg/100 ml. Total volatile fatty acid concentration and pH did not differ among treatments. Combinations of extruded soybeans and corn gluten meal were not advantageous compared with soybean meal as a supplemental protein source for lactating cows in this experiment.