The difficulty of transfecting primary macrophages and macrophage cell lines has meant that relatively few studies on regulation of gene expression have been performed in these cells. This study has optimized an electroporation procedure for the macrophage cell line RAW 264, but shows that introduction of DNA into the cytoplasm of primary macrophages by electroporation is toxic to the cells. It is proposed that this cell death may have a physiological role in defence against certain viral infections which result in accumulation of cytoplasmic DNA. RAW 264 cells were efficiently transfected by electroporation, but electroporated bone marrow derived macrophages (BMM) showed large scale cell death over a period of 12 h. Electroporation without DNA was not toxic and DNase treatment of samples before transfection prevented cell death. The toxicity of DNA was concentration-dependent and sequence-independent. Synthetic, genomic and plasmid DNA all caused cell death. This sensitivity to DNA seems to be distinct from the antiviral state induced by double-stranded RNA and may be part of an uncharacterized viral defence system.