The effects of 2 MeV neutrons were compared with those of 200 kV X-rays for the criterion of lethality in mastocytoma cells of ascites type. The cells prepared as suspension were exposed to the radiations in vitro under aerobic condition and transplanted in the abdominal cavity of mice. Their viability was estimated according to the growing speed of the excreted amount of urinary 5-HIAA in the host animal. The RBE varied between a value of 3.1 at the 10 per cent level of cell survival to 1.8 for the mean lethal dose (Do).