Human peripheral blood lymphocytes from 30 healthy donors aged from 5 to 65 years consisting of four groups, namely 5 (six males), 25 (six females), 45 (six males) and 65 years old (six males and six females), were exposed to a single adaptive dose of 0.01 Gy X-rays followed by a challenge dose of 2.0 Gy X-rays. All age groups responded with a significantly reduced incidence of chromatid and isochromatid breaks except Group 4 (65 years male and female groups), where adaptive and challenge doses did not significantly reduce chromatid and isochromatid breaks. Results indicate the possibility of an inducible chromosomal repair mechanism in human blood lymphocytes. It suggests that ageing could be a factor which abolishes the adaptive response.