The protective (antimutagenic) effect triggered by ‘conditioning’ treatment with low doses of mutagens, sublethal heat shock or compounds known as inducers of stress proteins prior to ‘challenge’ treatment with a high clastogen dose is described. Criterium of protection (‘clastogenic adaptation’) is the reduced quantity of chromatid aberrations induced by combination of ‘conditioning’ plus ‘challenge’ treatment as compared to only ‘challenge’ treatment of root tip meristems of higher plants (Vicia faba andHordeum vulgare). This anticlastogenic effect is absent in metabolically inactive cells and becomes counteracted by impairment of protein synthesis or inhibition of ‘G2 repair’. The data are indicative of at least 3 different inducible repair functions specific for certain types of DNA damage and reminiscent of those involved in the ‘adaptive response’ of bacterial and mammalian cells.