The protective effect of proflavine on gamma-irradiated bacteriophage lambda and its isolated DNA was investigated under conditions of predominantly indirect or direct effects. In both conditions addition of small amounts of the dye during irradiation of phage or DNA was shown to enhance their biological activity. Protection against indirect effects results probably from extensive scavenging of radioinduced water radicals within the medium. On the other hand the results obtained at minus 196 degrees C, with irradiated DNA-proflavine complexes, imply the existence of a long-range transfer of the primary radiation damage of DNA towards the intercalated molecules of proflavine. A mechanism for the protective effect of proflavine against the direct effect of ionizing radiation on biologically active DNA is suggested.