Experimental data from the author's laboratory are discussed, which demonstrate mutagenic and carcinogenic effects of low dose and low dose-rate exposure to ionizing radiation, and the modification of these effects by various chemical agents. These data are derived from studies of mutagenesis and neoplastic transformation in vitro, as well as on the induction of lung cancer in hamsters. On the basis of these results, several avenues of future research are suggested. These include: 1) the development and application of new human cell systems for studying radiation mutagenesis and transformation; 2) the study of the effects of protraction of exposure to radiation in such systems (emerging evidence suggests the dose-rate effect for mutagenesis may be minimal or absent in human cells); and 3) further examination of the role of secondary factors in radiation carcinogenesis. It appears unlikely that important information in these areas will arise from human epidemiologic studies. Knowledge derived from experimental systems should, however, enhance our understanding of the risks of radiation exposure and possible strategies to reduce them.