Human peripheral blood lymphocytes pre-exposed to low dose of Bleomycin which can induce double strand breaks in DNA, became less susceptible to the induction of chromatid breaks by high dose of same agent or by Mitomycin C which can induce cross-links in DNA, than those which were not pre-exposed to Bleomycin. But although lymphocytes adapted to Mitomycin C showed resistance to high concentration of the same agent, they did not show cross resistance to high concentration of Bleomycin. These results indicate that there might be different repair mechanisms in the adaptive response induced by different DNA damaging agents.