The spectrum of mutations induced by ionizing radiation (gamma-rays) was determined in the lacI gene of E. coli. Base substitution was the principal type of mutational event following ionizing radiation. Both transitions and transversions were produced, and no strong specificity for a particular base pair was observed. The spectra of spontaneous and of ionizing-radiation-induced base-pair changes differed significantly at several locations within the lacI gene. The location of 3 of these differences corresponded to sites of spontaneous deamination "hot spots" from which we conclude that gamma-rays do not cause extensive deamination. The specific locus rate was calculated as 4.5 X 10(-10) mutations per rad per gene copy per cell, and the nucleotide substitution rate was 2.2 X 10(-12) per rad. The frameshift mutation, trpE997, was not reverted by gamma-rays.