The three dimers 3, 4, and 5 of mitomycin C (MC), a natural antibiotic and cancer chemotherapeutic agent, were synthesized in which two MC molecules were linked with -(CH(2))(4)-, -(CH(2))(12)-, and -(CH(2))(3)N(CH(3))(CH(2))(3)- tethers, respectively. The dimeric mitomycins were designed to react as polyfunctional DNA alkylators, generating novel types of DNA damage. To test this design, their in vitro DNA alkylating and interstrand cross-linking (ICL) activities were studied in direct comparison with MC, which is itself an ICL agent. Evidence is presented that 3-5 multifunctionally alkylate and cross-link extracellular DNA and form DNA ICLs more efficiently than MC. Reductive activation, required for these activities, is catalyzed by the same reductases and chemical reductants that activate MC. Dimer 5, but not MC, cross-linked DNA under activation by low pH also. Sequence specificities of cross-linking of a 162-bp DNA fragment (tyrT DNA) by MC, 3, and 5 were determined using DPAGE. The dimers and MC cross-linked DNA with the same apparent CpG sequence specificity, but 5 exhibited much greater cross-linking efficacy than MC. Greatly enhanced regioselectivity of cross-linking to G.C rich regions by 5 relative to MC was observed, for which a mechanism unique to dimeric MCs is proposed. Covalent dG adducts of 5 with DNA were isolated and characterized by their UV and mass spectra. Tri- and tetrafunctional DNA adducts of 5 were detected. Although the dimers were generally less cytotoxic than MC, dimer 5 was highly and uniformly cytotoxic to all 60 human tumor cell cultures of the NCI screen. Its cytotoxicity to EMT6 tumor cells was enhanced under hypoxic conditions. These findings together verify the expected features of the MC dimers and warrant further study of the biological effects of dimer 5.