The role of repair of DNA double-strand breaks (dsb) in the determination of the RBE-value of alpha particles was studied using the temperature conditional radiosensitive diploid yeast mutant rad 54-3. This mutant is proficient in the repair of dsb at the permissive temperature of 23°C at which it yields a shouldered survival curve, but it is dsb repair-deficient at the restrictive temperature of 36°C at which it yields an exponential survival curve. At the permissive temperature the rad 54-3 mutant also shows liquid holding recovery of colony forming ability as a function of the liquid holding period. Thus, with this mutant it is possible to obtain survival curves involving no repair of dsb (immediate plating, 36°C), partial repair of dsb (immediate plating, 23°C) and gradually increasing levels of dsb repair by delayed plating after liquid holding periods of 24, 48 and 72 h. The RBE-values of densely ionizing 3.5 MeV alpha particles for cell killing relative to sparsely ionizing 30 MeV electrons have been determined as a function of the level of dsb repair. It is shown that the RBE-value is low and independent of dose when no repair of dsb is involved, whereas it becomes gradually larger with a gradual increase in the level of dsb repair.