Mitomycin C (MC), a cytotoxic anticancer drug and bifunctional DNA DNA alkylating agent, induces cross-linking of the complementary strands of DNA. The DNA interstrand cross-links (ICLs) are thought to be the critical cytotoxic lesions produced by MC. Decarbamoyl mitomycin C (DMC) has been regarded as a monofunctional mitomycin, incapable of causing ICLs. Paradoxically, DMC is slightly more toxic than MC to hypoxic EMT6 mouse mammary tumor cells as well as to CHO cells. To resolve this paradox, EMT6 cells were treated with MC or DMC under hypoxia at equimolar concentrations and the resulting DNA adducts were analyzed using HPLC and UV detection. MC treatment generated both intrastrand and interstrand cross-link adducts and four monoadducts, as shown previously. DMC generated two stereoisomeric monoadducts and two stereoisomeric ICL adducts, all of which were structurally characterized; one was identical with that formed with MC, the other was new and unique to DMC. Overall, adduct frequencies were strikingly higher (20-30-fold) with DMC than with MC. Although DMC monoadducts greatly exceeded DMC cross-link adducts ( approximately 10:1 ratio), the latter were equal or higher in number than the cross-link adducts from MC. DMC displayed a much higher monoadduct:cross-link ratio than MC. The similar cytotoxicities of the two drug show a correlation with their similar DNA cross-link adduct frequencies, but not with their total adduct or monoadduct frequencies. This provides specific experimental evidence that the ICLs rather than the monoadducts are critical factors in the cell death induced by MC. In vitro, overall alkylation of calf thymus DNA by DMC was much less efficient than by MC. Nevertheless, ICLs formed with DMC were clearly detectable. The chemical pathway of the cross-linking was shown to be analogous to that occurring with MC. These results also suggest that the differential sensitivity of Fanconi's Anemia cells to MC and DMC is related to factors other than a selective defect in cross-link repair.