A non-invasive electrorotation (ROT) technique was used to monitor the apoptosis-induced changes in HL-60 cells. The specific membrane capacitance of the cells fell from 15.6 +/- 0.9 mF/cm(2) stop to 6.4 +/- 0.6 mF/cm(2) stop after 48 h treatment with 10 nM bufalin, a component of bufadienolides in traditional Chinese medicine, Chan Su. However, the average membrane conductance remained almost constant during the first 24 h of treatment and then increased afterwards. Apoptosis was verified by a DNA fragmentation assay and scanning electron microscopy. The results demonstrate that the ROT technique gives a quantitative analysis of the toxic damage by chemicals to cells and can be exploited in the testing and development of new pharmaceuticals and active cell agents.