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Bleomycin, exclusively yet not necessarily, affects thymidine‐containing compounds such as DNA and polydeoxyribonucleotides by releasing free thymine and leaving aldehyde functions. Bleomycin action can be enhanced by adding reducing as well as oxidizing agents. Several radical scavangers were without influence. The guanidino moiety in the bleomycin could be traced as a functionally essential group. Molecular morphology and base sequence of the DNA strongly influence bleomycin activity. High bleomycin concentrations, besides modifying DNA into oligothyminic or athyminic nucleic acids, cause strand scissions. Possible reaction mechanisms are discussed.