The relative biological effectiveness was determined using sex-linked recessive lethals induced in Drosophila spermatozoa as the biological effect. The sex-linked recessive lethal test, a measure of mutations induced in germ cells and transmitted through successive generations, yields a linear dose-response curve in the range used in these experiments. A dose-response curve was determined from three exposures to tritiated water and three exposures to 60 Co γ radiation. The ratio of the slopes of these two response curves is 2.7 ± 0.3, yielding a relative biological effectiveness that suggests the tritium β particle is 2.7 times more effective per unit of energy absorbed in inducing gene mutations transmitted to successive generations than 60 Co γ radiation. The increase in relative biological effectiveness with higher linear energy transfer for tritium β radiation strongly suggests that single-strand breaks are repaired by a nearly error-free repair mechanism. Ion tracks with a high density of ions (high linear energy transfer) are more efficient than tracks with a low ion density (low linear energy transfer) in inducing transmissible mutations, suggesting interaction among products of ionization. Since most transmitted mutations induced by ionizing radiation result from strand breakage, interaction probably occurs at this level with double-strand breaks being repaired by an error-prone mechanism yielding transmissible mutations.