We plan to continue our studies of leukemogenesis in irradiated parabiont rats, including some pairs which have been separated about 6 to 8 weeks after radiation. Most of the single rats irradiated at higher dose levels (600 to 1100 R) have lived out their lifespans and have been studied. We plan to carry out radiation of the lower dose level groups (50 to 400 R). The results will be compared with those obtained from radiation dose-effect studies in mice. BIBLIOGRPHIC REFERENCE: Warren, S.: Effects on low levels of radiation on rodents and potential effects in man. Health Physics 29:251-255, 1975.