The bleomycin-mediated digestion of DNA in the presence of ferrous ion, molecular oxygen, and dithiothreitol is characterized by a fast initial reaction, which is followed by a much slower process. The fast degradation is due to the fast activation of the bleomycin-Fe(II) complex and the subsequent fast reaction of the activated complex with DNA. The rate determining step for the slow process is reactivation of the bleomycin-Fe(III) complex. The apparent rate constants for both reactions increase with increasing ionic strength. The latter, unusual results are interpreted in terms of inhibition of bleomycin turnover by binding of cationic species with DNA at low ionic strength.