The effect of dose fractionation on the radiation response of mouse tongue epithelium was quantified in fractionation protocols involving 1, 3, 4, 5 and 10 fractions, separated by at least 4 h. Fractionated irradiation was given either to the whole snout by 300 kV X-rays or locally to the tongue using 25 kV X-rays. Each protocol was terminated by a final local top-up dose (25 kV X-rays) of 5 Gy. The frequency of complete local denudation within the test area was used as the quantal end point. The kinetics of repair of sublethal damage was studied by snout irradiation with four equally spaced fractions, delivered at intervals of 35, 60, 90, 480 or 540 min, again followed by a local top-up dose of 5 Gy. The linear-quadratic model gave a satisfactory fit to the data with the exception of the four fraction/30-h data, suggesting cell cycle effects in this schedule. Analysis of the results with different two-step methods and with direct analysis yielded similar results. The alpha/beta ratio was determined to be approximately 11 GY (direct analysis: 11.6 Gy with 95% confidence limits of 8.1 and 16.4 Gy) and T1/2 was found to be 46 min (35-69 min). Both these values are in the range described for other acutely responding rodent tissues.