The lacI system of Escherichia coli provides a method for monitoring mutational events at a large number of sites. Using this system, we have previously determined the mutational spectra for γ-ray and β-particle emissions resulting from the decay of tritium. Analysis of these mutational spectra reveals that base substitution mutations induced by ionizing radiation are distributed nearly randomly throughout the lacI gene and include all detectable substitution events. The distribution of ionizing radiation-induced mutagenesis is similar to the low frequency of occurrence mutational events induced by other SOS-dependent mutagens. The lack of an apparent nonrandom or high frequency of occurrence component seen with other SOS-dependent mutagens can be best explained as the result of the random interaction of ionizing radiation with the DNA bases leading to production of a variety of base substitutions.