These experiments are designed to determine the mutagenic consequences of the introduction of a single DNA lesion into a known position of a DNA molecule. The system used is the highly defined M131ac forward mutation assay which allows detection and exact characterization of all classes of mutations both at and some distance away from the actual site of DNA damage. The first application of this approach, currently underway, is to determine the specificity of the muta-genesis resulting from the introduction of a single apurinic site into several individual positions in the DNA sequence which codes for the Alpha-complementing activity in the lacZ gene carried in M13mp2 phage. Mutants, produced under conditions of normal or altered DNA replication and/or DNA repair capacity, will be scored and defined by DNA sequence analysis. This approach is designed to overcome limitations to classical approaches which leave uncertainty as to whether the mutations recovered actually occur at the site of the lesion. Furthermore, an examination of these lesions at several locations also permit a determination of neighbouring base pair effects on the mutational specificity of such lesions. The results should provide substantial information on the specific mutagenic consequences of defined DNA lesions in biological systems.