Since the cytokinesis-block micronucleus assay is very sensitive at low radiation doses, we used it to investigate the in vitro sensitizing effects of two new hypoxic cell sensitizers (KU-2285, a fluorinated 2-nitroimidazole and RP-170, a 2-nitroimidazole nucleoside analogue) at 1-3 Gy in comparison with etanidazole. Exponentially growing EMT6 cells were treated with the drugs under aerobic or hypoxic conditions for 40 min prior to and during irradiation, after which the drugs were removed and cytochalasin B (2 micrograms/ml) was added to the medium. The number of micronuclei in binucleate cells was counted after 42 h of culture. Under aerobic conditions the three compounds at 5 mM had no sensitizing effect. Under hypoxic conditions the sensitizer enhancement ratio (SER) at 5 mM was 3.8 for KU-2285, 3.2 for RP-170, and 2.3 for etanidazole, while the oxygen enhancement ratio was 2.9. When the cells were pretreated under hypoxic conditions with drugs at 5 mM but then irradiated under aerobic conditions, KU-2285 and RP-170 had a sensitizing effect whereas etanidazole did not. The sensitizers were also tested at 0.5 and 1 mM, and the SER values were compared with those obtained at high doses (15-30 Gy) using a colony assay. The SER at low doses was higher than that at high doses for 1 and 5 mM KU-2285 and 5 mM RP-170, while the SERs were similar for all concentrations of etanidazole and the lower concentrations of KU-2285 and RP-170. These results might suggest the potential usefulness of KU-2285 and RP-170 in clinical radiotherapy.