Catalase and malate dehydrogenase (MDH) were subjected to the sound field produced by a transversely oscillating wire driven at 20 kHz. Catalase was not inactivated under any conditions of sonication whereas MDH inactivation increased exponentially with the duration of sonication and depended upon the initial enzyme concentration. The inactivation was not the result of collapse cavitation or thermal inactivation and was probably related to the presence of acoustic microstreaming.