Mature oocytes (class A) and immature oocytes (class B) from a repair-proficient yw strain and two repair-deficient strains (mus 302D1 and mei 41D5) were initially X-irradiated with 0.02 Gy and subsequently treated at various time intervals with 2 Gy. They were tested for dominant lethality (early and late death). For each strain and stage the observed values were below those expected on the basis of additivity of the effects of the two irradiations given separately. These results indicate the existence of a repair-stimulating effect of low doses for the repair-deficient strains as well as for the highly radiosensitive mature oocytes. This 'adaptive response' was highest for mei 41, lower for yw and lowest for mus 302, and also involved 'late death' lethality (real dominant mutations). The finding that the two repair-deficient strains showed significant differences in normal dominant lethality, as well as in radiosensitivity and adaptive response, is consistent with dissimilarities in the repair defect.