Unstable chromosome aberrations induced by in vitro irradiation with D-T neutrons have been analyzed in human blood lymphocytes. With respect to 250 kVP X rays a maximum limiting RBE at low doses of 4.1 was obtained for dicentric aberrations. Using aberrations as markers in mixed cultures of irradiated and unirradiated cells permits an assessment of interphase death plus mitotic delay. The low-dose RBE for this effect is 2.5. Assuming all unstable aberrations observed at metaphase would lead to cell death by nondisjunction allows an assessment of mitotic death. The low-dose RBE for this effect is 4.5. The data are compared with similar work obtained earlier with 242Cm α particles. The application of the present work to cytogenetic assessment of dose after accidental exposure to D-T neutrons is discussed.