The hair follicle or its differentiated product, the hair, which represents the linear historical record of the follicular proliferative activity, could provide a biological dosimeter of value for dose distribution determinations after accidental exposure. Here we present some further studies on irradiated mouse hair follicles and hair, and discuss the difficulties in obtaining similar data for humans. The incidence of cell death in the follicles has been shown elsewhere to be maximum 12 h after irradiation, and it increases with dose. Here we confirm that doses of 0.2-0.4 Gy can be readily detected. We show here that there is only a little more cell death in the larger follicles even though they contain many more cells and mitotic figures. About one-third of all the dead cell fragments in a follicle can be seen in a good longitudinal follicle section. Mitotic activity declines progressively with dose in the large follicles, which start with more mitotic cells, showing the dose-dependent changes most readily. The dead cells are morphologically identical to apoptotic cells at the level of the light microscope, and they fragment into several bodies, the number of which increases with dose. The total number of apoptotic bodies or fragments in whole large follicles increases almost 100-fold over a range of 1.3 Gy (from 0.2 to 1.5 Gy) and about tenfold over the range 0.2-0.5 Gy. The estimated number of dead (apoptotic) cells increases about sevenfold over the same 1.3-Gy range. The width of the middle portion of the broadest, awl, hairs measured 12 days after irradiation decreases with increasing dose. About 80% of the hairs show an obvious reduction in width after 2 Gy and the effects of a dose of about 1 Gy can be detected. The width of the hair is reduced by 10-14% per Gy. A comparison has been made between BDF1 (black) and BALB-c (albino) mice. The large follicles contain similar numbers of mitotic cells, but the BALB-c mice are more sensitive both in terms of the radiation-induced apoptosis and in terms of a reduction in awl hair width.