The antineoplastic drug daunorubicin and 15 other anthracyclines were tested for their ability to induce prophage lambda in Escherichia coli K12. Prophage lambda induction by daunorubicin was obtained in excision-repair deficient uvr- bacteria at doses about 3-fold lower than in excision-repair proficient uvr+ cells; this suggests that some of the lesions produced in DNA by daunorubicin are subject to excision repair and may be adducts. Daunorubicin seems to be converted to active species capable of causing prophage inducing lesions in DNA by bacterial enzymes. The antineoplastic and prophage inducing potencies of the anthracyclines were compared in a blind test. These two parameters were correlated for two thirds of the compounds. Such a correlation supports the idea that the antineoplastic activity of the anthracyclines is a consequence of their capacity to damage DNA.