Glutathione (GSH)-depletion by buthionine sulphoximine (BSO) altered both the aerobic and anaerobic radiation response of A549 human lung cancer cells grown in vitro. The oxygen enhancement ratio (o.e.r) was increased slightly from 3.0-3.3. The lack of an effect of GSH-depletion on o.e.r. reduction, provides a system whereby the mechanism of action of the thiol reactive reagent diethylmaleate (DEM) can be investigated. Pretreatment of cells with DEM, under non-toxic concentrations, removed 13 per cent of the intracellular NPSH and resulted in an o.e.r. of 2. When BSO followed by DEM was used, so that both GSH and NPSH were reduced to zero, an o.e.r. of 1.5 was obtained. Cells treated with 1 mM BSO for 24 hours contained 10 per cent NPSH and no GSH. When these cells were exposed to 0.5 or 1 mM DEM briefly, during irradiation, the o.e.r. was 2.4 and 1.7 respectively. In some cases altered o.e.r.s occurred in combination with increased aerobic responses. This was especially true for aerobic irradiations of BSO-treated cells in the presence or absence of DEM. However, the increased aerobic response was offset by a more dramatic increase in the hypoxic response. These results indicate (a) that GSH plays a significant role in aerobic radiation response but is not a principal factor in o.e.r.-reduction, and (b) that reduction of the o.e.r. by DEM is not due primarily to GSH-removal. The preferential radiosensitization of hypoxic cells by DEM may involve reactions of this compound with NPSH or protein SH, or may be related to the ability of DEM to mimic oxygen as a hypoxic cell radiosensitizer.