To determine whether the toxic effects and changes in many cell functions caused by antitumor 1-nitroacridines are related to their enzymatically mediated covalent interstrand DNA cross-linking (J. Konopa, J. W. Pawlak, and K. Pawlak. Chem.-Biol. Interact., 43: 175–197, 1983), the cross-linking potency of the derivatives with structural modifications in position 9 of the acridine nucleus was estimated as their in vitro threshold concentrations (0.3 to 4.5 µm), beyond which the first interstrand DNA cross-links could be detected in DNA of cultured HeLa S₃ cells with a polyethylene glycol 6000-Dextran T500 assay. Statistically significant (p < 0.05) correlations exist between the cross-linking potency of 1-nitroacridines and their in vivo antitumor activity and toxicity against mice with Sarcoma 180 tumors in solid form (3 to 1065 µmol/kg of body weight), as well as their in vitro cytotoxicity against cultured HeLa or HeLa S₃ cells (0.0005 to 7.2 µm), indicating that the interstrand DNA cross-linking potency might be one of primary determinants of in vivo and in vitro biological activity of 1-nitroacridine antineoplastic drugs. Susceptibility of the parent agents to reduction does not appear to be a rate-limiting factor of DNA cross-linking potency of 1-nitroacridines and their metabolic transformations (J. W. Pawlak, and J. Konopa. Biochem. Pharmacol., 28: 3391–3402, 1979), because no significant differences were observed among the agents with respect to their polarographic half-wave potentials estimated under anaerobic conditions.