Young adult Balb/c mice with hair follicles synchronously in the middle of the hair growth cycle received whole-body or partial-body doses of gamma-radiation or neutron radiation. The hair follicles were analysed either 3 days after irradiation in the dose-response experiments, or at various times after a constant dose in the time-course experiments, for changes in the number of cells in the forming medulla of the hair in the region just above the germinal matrix of the growing (anagen) hair follicle. Time-course experiments showed that 3 days after irradiating growing follicles (2 or 4 Gy of gamma-rays or 1 or 2 Gy of neutrons), the maximum reduction in the hair medullary cell count (HMCC) was observed. Survival curves were obtained for gamma-rays over a range of dose-rates (4.0-0.0023 Gy/min) using total doses between 0.5 and 5.0 Gy. A survival curve was also obtained for 62 MeV neutrons at a dose-rate of 0.31 Gy/min and doses of 0.1-2.0 Gy. The D0 for the HMCC dose-response curve following caesium gamma-irradiation at 4.0 Gy/min was 2.1 +/- 0.2 Gy with n = 1.7 +/- 0.2. The dose-response curve for low-dose-rate gamma-rays was best fitted by a simple exponential function with no evidence of a shoulder. The D0 was 3.0 +/- 0.1 Gy at a dose-rate of 0.04 Gy/min. The data for 62 MeV neutrons (at 0.35 Gy/min) had a small but significant shoulder n = 1.5 +/- 0.1 and a D0 of 1.0 +/- 0.1 Gy. These data further illustrate the sensitivity of this assay and its potential application as a biological dosimeter.