Plant DNA polymerases and E. coli DNA polymerase I, but not animal DNA polymerases or avian reverse transcriptase, are strongly stimulated by ethidium bromide (EtdBr) when TMP incorporation is followed using a short oligo dT primer at 37 degrees C. The effect is observed with a poly A template in the presence of Mg2+ or Mn2+ and of poly dA template only in the presence of magnesium ions. When a longer primer like poly dT is used, EdtBr inhibited wheat DNA polymerase C activity. This result prompted us to study the effect of the incubation temperature on the drug mediated stimulation. With oligo dT primer the stimulation by EdtBr is not observed at a temperature of incubation lower than 35 degrees C. It is shown that the Tm of poly A-dT12 is around 35 degrees C and that EdtBr will clearly increase this value. The stimulation is lost when the enzyme is preincubated with the primer alone whereas it is not affected when the enzyme is preincubated with the template.