We have investigated the effects of a single low dose of 42 rads neutron radiation on hair matrix cell kinetics. Anagen hair skin sites in 3-mo-old Carworth Farms no. 1 female mice were exposed to a fast neutron beam yielding a modal energy of 3.6 MeV. In contrast to low LET (linear energy transfer) radiation at a 2 1/2 fold higher dose, neutrons induced a markedly prolonged depression (greater than 60%) in the mitotic index for more than 18 hr postradiation. In addition, from 10--18 hr after neutron irradiation sharp elevations (greater than 80%) in the labeling indices apparently reflected significant impairment in cell maturation rates and/or slowed exit rates from the proliferative compartments. Postradiation reductions of up to 20% in the size of proliferating matrix cell populations were found. At the cellular level, this study confirms previous findings of a disproportionately high relative biological effectiveness for neutron radiation following low dose level exposure.