Supercoiled Col E1 DNA is split by [Escherichia coli] EcoRI endonuclease at 37.degree. C without intermediate formation of open circular DNA. Accumulation of this restriction product is observed at low temperature. The fluorescent dye, 4,6''-diamidino-2-phenylindole (DAPI) inhibits restriction by EcoRI endonuclease. This effect is due to the DAPI:DNA rather than to the DAPI:EcoRI interactions.