In cell culture the cytotoxicity of some anticancer drugs, especially bleomycin, can be greatly enhanced by exposing cells to non-cytotoxic electric pulses. Nude or conventional mice bearing subcutaneous transplanted tumours were treated with intramuscular doses of bleomycin followed by local delivery of electric pulses similar to those used in vitro. Tumors were reduced and even eradicated after this electrochemotherapy. Thus the antitumour effects of bleomycin in mice can be considerably potentiated by local electric pulses.