Summary The hypoxic cell radiosensitizer RSU 1069 (1-(2-nitro-1-imidazolyl)-3-(1-aziridinyl)-2-propanol) shows, on a concentration basis, a 100-fold greater toxicity towards hypoxic relative to aerobic cells. This toxicity is substantially greater than that of misonidazole, a compound of similar electron affinity. Reductive processes are important for hypoxic toxicity; this is demonstrated by the fact that misonidazole, in excess, can protect against the hypoxic but not aerobic toxicity of RSU 1069. The importance of the interaction of RSU 1069 with DNA, suggested initially by molecular studies, is supported by the fact that cells containing 5-bromodeoxyuridine (5-BUdR) incorporated into their DNA show greater sensitivity towards the lethal effects of RSU 1069 both in air and nitrogen, compared to cells not treated with 5-BUdR. Experiments with RSU 1069 and 3-aminobenzamide (3-AB) show the latter compound to potentiate aerobic toxicity, consistent with monofunctional alkylation by RSU 1069. In contrast, 3-AB has no effect on the hypoxic cytotoxicity of RSU 1069, which would be predicted if RSU 1069 is functioning as a bifunctional agent under these conditions. It is our contention that in air, RSU 1069 functions as a typical monofunctional alkylating agent, presumably due to the presence of the aziridine group whereas, in hypoxia, reduction of the nitro group provides an additional alkylating species, converting the compound into a bifunctional agent.The compound RSU 1069 (NSC 347503), 1-(2-nitro-l-imidazolyl)-3-(1-aziridinyl)-2-propanol, has a similar one-electron reduction potential to that of misonidazole (Adams, 1984a). However it is both much more efficient as a radiosensitizer and considerably more cytotoxic than misonidazole both in vitro and in vivo (Adams et al., 1984a,b). Structurally RSU 1069 differs from misonidazole in that an aziridine replaces the methoxy group in the NI side-chain of the 2-nitroimidazole. Aziridines are monofunctional alkylating agents which can react with cellular macromolecules such as DNA (see e.g. Ross, 1962) and it has been recently shown that RSU 1069 and a product(s) of its reduction can bind to calf thymus DNA and cause single strand breaks in plasmid DNA (Silver et al., 1985).The aim of the present work was to characterize the cytotoxic effect of RSU 1069 at the cellular level. Results will be discussed with regard to those obtained in molecular studies (Silver et al., 1985;Silver & O'Neill, 1986)