The relative biological effectiveness (RBE) for the induction of DNA strand breaks and the efficiency of repair of these breaks in cultured diploid bovine lens epithelial cells was measured, using accelerated heavy ions in the linear energy transfer (LET)-range up to 16,200 keV/micron. At LET values above 800 keV/micron, the number of DNA strand breaks induced per particle increases both with the atomic number of the projectile and with its kinetic energy. About 90 per cent or more of the strand breaks induced by ions with an LET of less than 10,000 keV/micron are repaired within 24 h. Repair kinetics show a dependence on the particle fluence (irradiation dose). At higher particle fluences a higher proportion of non-rejoined breaks is found, even after prolonged periods of incubation. At any LET value, repair is much slower after heavy-ion exposure than after X-irradiation. This is especially true for low energetic particles with a very high local density of energy deposition within the particle track. At the highest LET value (16,200 keV/micron), no significant repair is observed.