. In addition, a potential role for its use in chemotherapy is being considered because of the ability of this drug to be preferentially cytotoxic towards hypoxic cells (Hall & Roizin-Towle, 1975;Moore et al., 1976;Sridhar et al., 1976;Stratford & Adams, 1977 foetal calf serum (FCS, Gibco-Biocult Ltd.). Cells were kept in log phase at concentrations ranging between 105 and 106/ml. Cytotoxicity experiments.-250ml spinner flasks were fitted with a gas inlet/outlet system and a sidearm through which samples could be withdrawn. Asynchronous, logphase cells at a concentration of 2 x 105/ml were suspended in MEM + 7.5 % FCS and held in a water bath at 37TC. The compounds were dissolved in MEM+7.5% FCS and added to the suspension which was buffered with bicarbonate to pH 7-4. When appropriate, the spinner containing cells was deaerated by flowing N2 plus 5% CO2 (<10 pts/106 02; BOC Ltd) at 500 ml/min over the surface of the stirred suspension throughout the experiment. Samples of cells were withdrawn at appropriate times, centrifuged, resuspended, counted, diluted, plated in MEM+ 15% FCS and incubated for 7