Tumor hypoxia provides a key difference between healthy and cancerous cells. It can be exploited to produce drug selectivity, offering a reductase-rich environment for prodrug activation. Nitrogen mustard drugs are cytotoxic, but usually unselective. Polyamine mustards are candidates for conversion into hypoxia-selective prodrugs via complexation with metals. Reduction to a less stable complex can free the active drug. The novel Cu(II) complexes of N-mustard derivatives of 1,4,7-triazacyclononane (tacn), 1,4,7,10-tetraazacyclododecane (cyclen), and 1,4,8,11-tetraazacyclotetradecane (cyclam) were assessed in vitro as hypoxia-selective cytotoxins. The cyclen mustard complex showed 24-fold selectivity as a hypoxia-selective bioreductive prodrug, with an IC50 value of 2 μM against the lung tumor cell line A549. Reversible redox behavior and stability of the cyclen-Cu(II) complex in aqueous solution correlated with good hypoxia selectivity. The two other related complexes showed irreversible redox behavior and low aqueous stability and were not hypoxia-selective. The use of macrocyclic nitrogen mustard complexes represents a promising new strategy in the design of hypoxia-selective cytotoxins.