In a recent report we described the effects of combined electroactivation and photoactivation of hematoporphyrin derivative (HPD) on human erythrocytes and established that activation-induced cell lysis was more pronounced when both modes of activation were sequentially applied to the system. Here we demonstrate that electric field-induced activation of HPD-treated HeLa cells results in cell death, This effect is shown to be dependant on both electric field strength and on HPD concentration. In addition, we demonstrate that exposure of HPD-treated cells to shea and intense electric pulses prior to photoactivation, results in increased cell mortality. The results confirm our earlier suggestion that HPD may be activated in the presence of an applied electric field, The results further suggest that activation of photosensitizers using combined exposure to electric fields and light may play an important role in increasing the efficiency of photodynamic therapy (PDT) in the treatment of cancer. (C) 1997 Elsevier Science Ireland Ltd.