The thermal degradation of plasmid pUC18 held at temperatures between 100 and 135 °C was examined by measuring the ability of heat-treated plasmid preparations to transform Escherichia coli to ampicillin resistance using electroporation. Substantial protection against loss of transforming ability during heating was provided by concentrations of NaCl between 0.25 and 2.0 mol l−1. For example, the addition of 1.0 mol l−1 NaCl to samples heated at 100 °C for 15 min increased transformation frequency about 200-fold compared with samples heated without NaCl. In the presence of 0.5-2.0 mol l−1 NaCl, transforming capacity was not destroyed even by heating at 121 °C for 15 min, i.e. after a typical sterilization treatment. These findings may have implications for the safe disposal of genetically modified micro-organisms and recombinant DNA preparations.