The relative biological effectiveness (r.b.e.) of cyclotron-produced fast neutrons (11 MeV) in relation to 60Co gamma-rays, was studied in a wild-type and a DNA repair-deficient yeast strain for cell killing and genotoxicity. In the wild-type (D7) strain the r.b.e. varied from 2.7 to 4.1 for lethality, 2.8 to 7.1 for reverse mutation and 3.5 to 7.8 for mitotic gene conversion. At different survival levels, the repair deficient strain (D7 rad 52/rad 52) generally showed a lower r.b.e. for both cell killing and genotoxicity (25.2 to 37.2 per cent reduction for the cell death and 24.8 to 70.6 per cent for mutation and gene conversion) compared to the wild type. Except at very low dose levels, the r.b.e. values for cell killing and genotoxicity were similar within a given strain. At similar survival levels, neutrons were no more genotoxic than gamma-rays.