Summary
The frequency of hypoxanthine phosphoribosyl transferase (HPRT) deficient splenic T lymphocytes was measured in the 137Cs γ-irradiated mouse by the T cell cloning method. Doses from 0·3 to 6 Gy were applied at the dose-rates 0·5 Gy/min, 1 Gy/day and 1 Gy/week. Mutants were determined 8–10 and 30–40 weeks after the end of exposure. Radiation-induced mutant frequency (MFi) was calculated by subtracting the age corrected spontaneous mutant frequency (MFsp) from total mutant frequency (MF) found in irradiated animals. Data were fitted to linear and linear—quadratic dose—response models. MFi depended markedly on dose, dose-rate and time after exposure. When mutants were determined 8–10 weeks after acute irradiation (0·5 Gy/min) the dose—effect curve fitted the linear—quadratic equation MFi = 6·9 × 10−6 Gy + 1·2 × 10−6 Gy2, whereas in low dose-rate experiments (1 Gy/day, 1 Gy/week) the dose—effect curves were linear. The slope of the linear regression was about 3 × 10−6. When low dose-rate-irradiated animals were killed 30–40 weeks after irradiation, MFi was about one-third of that observed after 8 weeks. The dose dose-rate effectiveness factor (DDREF) for radiation mutagenicity was calculated in animals that had been exposed 8–10 weeks previously. For doses < 2 Gy the reduction in effectiveness was about 1·5 when the irradiation dose-rate was ąron; 1 Gy/day. For higher doses DDREF was 3–5.