An optimal time of 45 min was established for extraction of rennin (chymosin) from abomasa using ultrasound. Effect of swelling of raw material and increased temperature of extraction medium (45 degrees C) was established. As rennin is resistant to ultrasound treatment and enhanced temperatures, ultrasound extraction results in a significant increase in the yield and activity. However, purified rennins were sensitive to ultrasound action. There were found changes of the electrophoretic properties and amino acid composition. Physicochemical properties were different and quality characteristics were higher for ultrasonic extracted rennin than for the control, particularly specific clotting activity, proteolytic activity, and rheological properties of curd. However, nonspecific proteolysis, coefficient of rennin effectivity, appearance, odor, solubility, moisture, protein, and NaCI content were similar. Ultrasonic extraction caused a decrease in the total viable counts of microorganisms of rennin, because of the short time of extraction.