Several significant effects of ultrasonic treatment on human serum cholinesterase and aminopeptidase isoenzymes and on other serum proteins have been found by starch gel electrophoresis. The selective activation of one cholinesterase isoenzyme is especially striking. These effects must be considered when ultrasonic treatment is used for the extraction of intracellular enzymes. When the effects are appreciated, ultrasonics should provide a valuable tool for isoenzyme research.