The dominant mechanism for microbial cell disruption in a high-pressure homogenizer and a high-speed homogenizer used in this study has been identified. It was found that the cavity collapse and the pressure pulse resulting from such a collapse have a significant influence on the rates of cell disruption. The similarities among the operating conditions for the decomposition of the aqueous KI solution to liberate iodine, the reaction occurring only under cavitating conditions, and that required for the substantial disruption of microbial cells have been pointed out. The liberation of iodine by the aqueous KI decomposition is treated as evidence of cavitation, and hence microbial cell disruption occurring at an identical discharge pressure confirms the mechanism of cell disruption as cavitation, in the high-pressure homogenizer valve. In the case of the high-speed homogenizer, shear and cavitation both play a significant role in cell disruption.