Hyperthermic treatment of HeLa cells in suspension combined with ultrasound irradiation produced alterations to the cell surfaces. The changes induced were related to ultrasound intensity in the standing wave and to heat treatments between 37 and 45 degrees C. Two transducers were used, driven at resonant frequencies of 0.75 and 1.5 MHz, and producing peak intensities up to 7 W/cm2. These intensities produced a negligible rise in temperature of the cell suspension medium. Ultrastructural damage in standing wave fields, as seen by scanning electron microscopy, progressed through stages. The first stage was characterized by the loss of microvilli and smooth appearance of the cell surface, e.g. after insonation at 41.5 degrees C for 10 min; damage increased to a final stage where the surface appeared heavily pitted and porous, with the cells showing signs of disintegration, e.g. after insonation at 45 degrees C for 10 min. The monitoring of ultrasound-induced cavitation suggested that damage was caused by bubble oscillations, not collapse cavitation. Shearing stresses accentuated by hyperthermia were considered the probable cause of such damage. Coulter counter studies of cell size distribution showed that the extent of cell damage depended on the geometry of the vessel in which insonation was carried out.