The sources of iron (Fe) and reductant required for DNA strand breakage by the antitumor drug bleomycin (Blm), H2O2 and ascorbate were investigated using nuclei instead of whole cells in order to study a simpler, related system that was subject to better control and easier chemical manipulation. Ehrlich ascites tumor cells were isolated and treated directly on filters, and analysed for DNA damage by alkaline and nondenaturing elution. Extraction and treatment buffers were depleted of trace Fe by passage through Mg(OH)2 gel. Nuclei were treated for 1 hr at 37 degrees. High levels of single- and double-strand breakage were obtained using Fe(III)Blm in the range 0.01 to 0.08 microM. In contrast, Blm was effective only at two orders of magnitude greater concentration. Cu(II)Blm was totally ineffective in causing damage. Depletion of nuclear protein thiols with N-ethylmaleimide reduced double-strand breakage at the upper end of the FeBlm concentration-response curve. A 1 mM concentration of NADPH or NADH greatly increased the extent of double-strand breakage by 0.01 microM FeBlm, suggesting roles for cytochrome P450 or cytochrome b5 reductase in strand breakage. Fe(III)ATP (1:20 metal to ligand and 50 microM in Fe) and Fe(III)EDTA (1:2 metal to ligand and 50 microM in Fe) did not cause single-strand breaks. In the absence of added Fe, H2O2 or ascorbic acid (50 microM) caused less than one Gy-equivalent single-strand breakage. Addition of ascorbate plus Fe(III)ATP or Fe(III)EDTA produced breakage beyond the capacity of alkaline elution to analyse (5-6 Gy). Overall, the results indicate that Fe, which may contribute to DNA damage by Blm and forms of activated oxygen within cells, is not strongly bound in the nucleus and that nuclear thiols other than glutathione contribute reducing equivalents to Fe(III)Blm for the DNA damaging chemistry.