Radiation and radiomimetic chemicals can be both carcinostatic and also carcinogenic and mutagenic. In all cases the critical reaction is with the cellular DNA in which both ionizing radiation and radiomimetic chemicals produce a variety of adducts and changes. Human cells respond to these lesions in several ways. Some adducts are ignored. Other are recognized as different by the excision repair mechanism and are cut out of the DNA. Other adducts may be by-passed by special post-replication repair mechanisms so that viable daughter cells still containing altered DNA are produced. Unrepaired lesions may lead to chromosome aberrations and cell death. Since only viable cells can produce tumors, post replication repair is critical to the initial events in carcinogenesis. Lesions which are converted to DNA strand breaks, on the other hand, lead to cell death. Knowledge of the changes produced in DNA and understanding of the different cellular responses possible should permit prediction of the relative tumorigenic and tumoristatic properties of compounds.