The effects of ultrasound on the rates of the catalytic reactions of four purified enzymes in vitro have been extensively investigated under a wide range of biochemical and physical exposure conditions. In general, it can be concluded that therapeutic intensities of continuous wave 0.88 MHz ultrasound had no detectable direct effects on the rates of the reactions catalysed by creatine kinase, lactate dehydrogenase, hexokinase and pyruvate kinase. Some minor effects were noted. These were: an indirect effect resulting from mixing within the sample chamber caused by quartz wind streaming; an effect on partially-hydrated cross-linked enzyme systems which appears to be the result of increased fluid penetration of the solid matrix in the presence of ultrasound; and an increase in the rate of spontaneous dissociation of a multimeric enzyme system. It is, therefore, concluded that a direct interaction between ultrasound and the catalytic functioning of individual enzyme molecules is unlikely to be the primary step in any acousto-biological interaction, and that this primary interaction appears to be occurring at a higher level of organizational complexity.