Jejunal crypt survival after fractionated total body irradiation of C3H mice given at dose rates of 1·2 or 0·08 Gy/min was studied. The fractionation effect was more pronounced at the high dose rate than at the low dose rate. Analysis of the data according to the linear-quadratic survival curve model yielded an αbeta; value at 1·2 Gy/min of 13·3 Gy and at 0·08 Gy/min of 96 Gy.