A review of the literature on ionizing radiation mutagenesis (IRM) in Escherichia coli indicates that many important questions remain unanswered. We propose to answer a few of these. (1) Why does IRM show a linear dose-response with some mutation assays, and a nonlinear dose response with other assays, or with the same assay but different growth conditions? Is this evidence for multiple mechanisms for IRM? (2) Why does a lexA mutation completely block UV radiation mutagenesis (UVRM), but not IRM? Does this suggest a difference in the mechanisms for UVRM and IRM? (3) Does ionizing radiation produce frameshift mutations? Two recent papers on this subject are in conflict. (4) Does IRM involve heteroduplex repair (i.e., a type of excision repair whereby heteroduplex DNA is converted to homoduplex DNA)? This hypothesis is suggested to explain why IRM (supposedly a one-hit phenomenon) shows both strands of the DNA duplex being mutated simultaneously. (5) What is the mechanism by which ionizing radiation produces mutations that can immediately express their phenotype (a phenomenon not generally characteristic of UVRM)? (6) Three postirradiation treatments that reduce the yield of IRM will be characterized in an attempt to explain their molecular bases, e.g., are they the result of error-free DNA repair processes? Health Relatedness: This research proposal suggests procedures for elucidating the molecular basis of IRM. Since the association between mutagenesis and carcinogenesis is now generally accepted, this research should also provide insights into the molecular mechanism of ionizing radiation carcionogenesis. The biological effects of low levels of ionizing radiation can be accurately determined in E. coli because of the ease of studying large numbers of individuals.