Ultrasonic damage to cells in vitro has been reported to vary dramatically with cell density; the particle density per se has been proposed to be responsible for this effect via interference with gas body activity. Reported here are the results of experiments designed to test the general postulate that the "cell density effect" can be explained in part by cellular modification of the suspension medium by respiratory gas exchange. Ultrasonic cell lysis is shown to occur at a much higher level in suspensions supplemented with the respiratory inhibitor NaCN than in control suspensions when suspension densities exceed 2 x 10(7) cells/mL. Moreover, at constant cell density, cell lysis diminishes with increasing pre-insonation incubation time at 37 degrees C. The rate of change in cell lysis with incubation time was diminished significantly by cyanide treatment. These observations are consistent with the postulate that respiratory O2:CO2 exchange enriches the medium in CO2 while depleting the medium of oxygen, which cavitates more readily than CO2, thereby diminishing the potential for cavitation-related cellular damage. However, this is only a partial explanation of the cell concentration dependence of cell lysis; cell density per se is an important factor in the "cell density effect."