Free radicals created in glycylglycine and acetylglycine by ionizing radiation have been reported by several investigators (1-4). Using ESR, these investigators observed an anisotropic doublet absorption in irradiated single crystals of these compounds. The absorption in acetylglycine was attributed (1, 4) to the radical formed by the removal of a methylene proton from the a-carbon. In glycylglycine, also, the radical was thought (1-3) to be the result of a proton removed from the a-carbon at the acid end of the molecule. The doublet absorptions, however, were broad-for example, a 7-gauss half-width for each component of the doublet in glycylglycine (1). The breadth of line implied unresolved interactions and, therefore, limited the accuracy and the meaningfulness of the hyperfine tensor elements derived from these spectra. In the present investigation, greater resolution is achieved by utilizing the technique of zero-field magnetic resonance. The zero-field results suggest that the doublet observed in ESR is a composite of absorptions attributed to two similar radicals. Evidence is presented which implies that these radicals are associated with damage sites on the a-carbon near the acid end of the molecule. The zero-field spectra of irradiated glycylglycine hydrochloride and glycylglycylglycine were also measured. The spectra of these structurally related compounds resemble those of glycylglycine and acetylglycine. Ionizing radiation appears to form closely related free radicals in all four peptides.