This paper describes the radiosensitizing effects of diatrizoic (DA) and iothalamic (ITA) acids and of iodipamide (IP) on the survival of E coli B/r irradiated with X-rays and with high-intensity electron pulses. All compounds at concentrations between 10 and 50 mM display a strong sensitizing effect in the presence of oxygen (DMF between 0-1 and 0-3) and are much less effective in nitrogen. In N2O the degree of sensitization is intermediate between oxygen and nitrogen. The situation is the same at pH 7 or 5-6. Solutions of DA, ITA and IP irradiated at pH lower than 6 become highly toxic to bacteria added after irradiation, for several hours after X-irradiation or several minutes after pulsed irradiation. The maximum toxic effect occurs with 2 krad of X-ray and with 6-8 krad of electrons. Oxygen must be present in order to observe the bactericidal activity. This is not affected by scavenging hydrated electrons with nitrate, but is completely cancelled by scavenging OH radicals with formate. It is also cancelled by adding thiosulphate to the irradiated solutions immediately before the bacteria. In the presence of nutrient broth, the radiosensitizing effect is absent after irradiation with pulsed electrons; whereas after X-irradiation it is reduced when the concentration of sensitizers is 50 mM. The experimental data appear to be compatible with a mechanism operated by short and long-lived transients resulting from the radiolysis of iodinated contrast media.