We provide initial electron nuclear double resonance (ENDOR) findings on the electronic structure of iron-containing bleomycin and its activated oxygen. Q-Band (34 GHz) ENDOR has resolved hyperfine couplings from the activated O-17 oxygen that originates in isotopically enriched O-17(2) gas. The largest O-17 hyperfine coupling, approximately 27 MHz, was best resolved at an intermediate g-value (g(y) 2.17), and a smaller hyperfine coupling, approximately 10 MHz, was resolved at the extremal g(z) (g(max)) and g(x) (g(min)) features as well as at g(y). The wider frequency range and anisotropy of O-17 ENDOR features from activated oxygen in bleomycin were in marked contrast to the narrow ENDOR feature of the activated oxyferryl oxygen of horseradish peroxidase compound I (Roberts, J. E.; Hoffman, B. M.; Rutter, R.; Hager, L. P. J. Am Chem. Soc. 1981, 103, 7654-7656). The implication of this contrast is that the electronic structure of the activated oxygen differs markedly in the two compounds. The agreement between Q-band ENDOR-resolved hyperfine couplings and hyperfine couplings estimated from X-band EPR line broadening at g(x) suggests that there is a large hyperfine coupling to only one oxygen. Hydroperoxide has been indicated as an iron ligand for activated bleomycin (Sam, J. FV.; Tang, X-J.; Peisach, J. J. Am. Chem. Sec. 1994, 116, 5250-5256), and strong hyperfine coupling to one oxygen is compatible with end-on ligation of the hydroperoxide. Exchangeable proton ENDOR patterns, well separated at Q-band from N-14 and O-17 features, had maximal couplings of the order 13 MHz near the g(z) axis, as might be expected for protons which are hydrogen bonded to first shell amine nitrogen or oxygen axial ligands. Activated bleomycin and ferric bleomycin (prepared by chelating ferric ion with bleomycin) differed in details of their proton ENDOR. Nitrogen (N-14) ENDOR features and hyperfine couplings resembled those of low-spin ferric heme and imidazole nitrogens.