The radiosensitization of two human tumour in vitro cell lines, HT-1080 and LoVo, has been compared with that of the Chinese hamster cell line V79-379A. Although the two human tumour cell lines were more radiosensitive than the V79 cell line sensitizer, enhancement ratios for misonidazole, pimonidazole and azomycin were similar (relative to extracellular concentration) for all three cell lines. Average intracellular concentrations of radiosensitizer were measured by high-performance liquid chromatography. In all three cell lines the uptake of misonidazole and azomycin was extremely rapid whereas that of pimonidazole was initially much slower before reaching a plateau. The ratios of intracellular concentration of radiosensitizer to extracellular concentration (Ci to Ce) for misonidazole were 0.8 (HT-1080) and 0.7 (LoVo and V79); for azomycin 0.9 (HT-1080 and LoVo) and 0.8 (V79). In contrast Ci/Ce for pimonidazole varied with cell line, the values being 1.8 (LoVo), 2.6 (HT-1080) and 3.3 (V79). Intracellular amounts of non-protein sulphydryl (NPSH) varied between cell lines by about a factor of three. However, when the average cell volume was taken into consideration the concentrations of NPSH were very similar, being 4.2 (HT-1080), 5.6 (LoVo) and 5.7 (V79) mmol dm-3. NPSH levels expressed as nmol per mg protein were also similar.