Ultrafiltered skim milk concentrated to 20-22% solids was hydrolyzed with a mixture of proteases from Aspergillus oryzae and an acid protease from Aspergillus niger. The enzyme preparation from A. oryzae did not produce bitterness, so the effects of its hydrolyzing action on the whipping properties of the retentate in liquid and powdered forms were examined. Its effect on the amount of soluble nitrogen in the powder was also determined. After inoculation of 250 ml retentate with .05% w/v enzyme, the samples were held in a 45°C water bath. The samples thickened with increasing hydrolysis until syneresis occurred. They also took on a slightly ropy or slimy appearance after thickening and before syneresis. Hydrolysis was stopped by removing from the 45°C bath and holding in an 80°C bath for 35 minutes. When cooled to refrigerator temperature, retentate samples produced a stable whip with fine bubbles and firm but soft peaks. Air capacity of whipped samples decreased with increasing hydrolysis time and retentate solids. Stability increased with increasing solids and longer hydrolysis times. Skim milk retentate to be dried was concentrated to 20-24% solids and reconstituted to 20% before whipping. Hydrolysis time up to 20 minutes before enzyme denaturation did not have an effect on the air capacity of the whipped reconstituted powder. Air capacity decreased as the retentate solids increased. Whip stability increased slightly then decreased with increasing hydrolysis. Whip stability was impaired by protein denaturation in the powder during the drying process. Soluble nitrogen in the powder, as determined by the Harland-Ashworth procedure, increased with increasing hydrolysis and decreased with increasing solids.