The effects of ultrasound on the cell membrane of HeLa cells have been investigated using 750 KHz and 1.5 MHz transducers. The insonation chamber was designed to present minimum perturbance to ultrasound, while a castor-oil filled cylinder was used to reduce unwanted reflections and subsequent interference. Cells were insonated in monolayers and cell suspensions using continuous wave ultrasound with peak intensities up to 7 W/cm² for both transducers and temperatures up to 45^oC. The insonations were carried out in progressive wave and standing wave fields. Scanning Electron Microscopy revealed damage to cell membrane of cells insonated in suspension in the standing wave fields. No damage was inflicted on HeLa cells in the monolayers which were insonated under otherwise identical conditions. Furthermore, the damage was absent in both the cell suspensions and monolayers when insonated in the progressive wave fields. The observed damage progressed through distinct stages and it was temperature and intensity dependent. Cavitation monitoring indicated the presence of oscillating gas bubbles rather than collapse cavitation. In general, all these findings have given support to shearing stresses as the likely mechanism responsible for the damage. The severity of damage produced in the HeLa cell membrane strengthens the view held by other workers that the cell membrane is a major target for ultrasound damage. Coulter Counter study has also demonstrated the dependence of cell damage on the geometry of the container and highlights the importance of specifying the precise insonation conditions. ESR spin labelling technique has been employed in this study to probe the ultrasound effect on the fluidity of cell membrane. This preliminary investigation indicated that ultrasound increases the fluidity of the cell membrane. The implication of this increase in fluidity has been discussed.