Chemical investigations leading to the construction of bis(bioreductive) alkylating agents having both conformationally restricted and mobile spacer regions are described. Two targets having the conformationally mobile ethylene spacer group, namely, 2,2'-ethylenebis[6-(hydroxymethyl)-p-benzoquinone] diacetate (3b) and 2,2'-ethylenebis[6-(bromomethyl)-p-benzoquinone] (3c), were studied in vivo and in vitro using an established epithelial/Burkitt lymphoma hybrid cell line (D98/HR1) previously shown to induce carcinomas in nude mice. Inactivity of both test compounds in vitro, the relative resistance of these cells to test drugs in vitro, and the selective antitumor properties of the bis(bromomethyl) analogue in vivo lead to the proposal that this compound undergoes bioreduction to an alkylating species in the hypoxic core of the tumor, thereby exerting its action.