In the present paper, we report data on the possible adaptive response, induced in vivo by exposure to ionizing radiation to a challenge treatment with the radiomimetic glycopeptide bleomycin (BLM). Lymphocytes from children living in Pripjat at the time of the Chernobyl accident, and thus hit by the initial acute dose of ionizing radiation, were treated for the last 5 h of culture with 0.004 U/ml BLM. Significantly lower chromosome damage was found only in lymphocytes from children who, independently of the initial acute exposure to ionizing radiation, still showed a 137Cs internal contamination, due to persistent continuous exposure to low doses of radiation. The present results indicate that past exposure to acute high dose of ionizing radiation does not interfere with resistance to BLM which is related to internal contamination.