Cell toxicity by BZQ could not be explained by free radical formation and thus further work has been undertaken to elucidate a possible mechanism of cell killing. By using radiolabelled BZQ, in vitro DNA-drug binding has been investigated. The effect of salt, buffer and drug concentrations was determined in the pH range 4.0 to 8.0. The influence of in situ oxidation and reduction on BZQ binding was also studied as a function of pH. In an effort to ascertain any base specificity of BZQ binding the homopolymers, Poly[dG]. Poly[dC] and Poly[dA]. Poly[dT] were treated with radiolabelled BZQ in the pH range 4.0 to 8.0. A fluorescence assay was used to demonstrate the possible involvement of DNA cross-linking in cellular activity. From this work, it was concluded that BZQ functions as a bifunctional alkylating agent by an acid-assisted aziridine ring-opening mechanism and that other factors including oxidation or reduction are much less important.