The cytotoxicity, metabolism, and DNA alkylation of porfiromycin (PFM) under aerobic and hypoxic conditions were evaluated in P388 murine leukemia cells. Clonogenic assays showed that the IC50 value for a 1-h exposure to PFM was 4 microM for aerobic cells and 0.5 microM for hypoxic cells. After a 1-h exposure to concentrations of 1, 5, and 10 microM [14C]-PFM, the accumulation of total radioactivity in hypoxic cells was 10 to 20 times that in aerobic cells. The disposition of radioactivity in cells that had been treated for 1 h with 5 microM PFM under aerobic or hypoxic conditions showed that (a) under either condition, internal free-PFM concentration equalled the external drug concentration; (b) DNA-, RNA-, and protein-bound radioactivity were at least 10 times greater in hypoxic cells than in aerobic cells; and (c) known metabolites and unidentified radioactive products were also generated in greater amounts in hypoxic cells than in aerobic cells. Thus, the increased amounts of radioactivity accumulated by hypoxic P388 cells after exposure to [14C]-PFM resulted from the accumulation of nonexchangeable protein and nucleic-acid adducts and metabolites rather than free PFM. Determinations of DNA adducts formed in P388 cells revealed five possible adducts: (1) N2-(2'-deoxyguanosyl)-7-methylaminomitosene, (2) a second monofunctional PFM-guanine adduct, (3) a PFM cross-linked dinucleotide, (4) possibly a nucleoprotein-related adduct, and (5) an unknown. We conclude that the enhancement of PFM-induced cytotoxicity by hypoxia appears to be primarily due to increased alkylation of macromolecules.