Mouse bladders were irradiated with 1,2,4 or 8 fractions of electrons or neutrons. Urination frequency was scored from 9 to 14 months after irradiation and this was used as a measure of the extent of bladder damage. The RBE for neutrons relative to 1.8 MeV electrons increased from 3.2 for large single doses to 5.4 for 8 fractions. Repair of sublethal injury after electron irradiation was considerable, with 63-72% of each fractionated dose repaired. Repair after neutron irradiation was less, with only 35-43% of each fractionated dose recovered. Linear-quadratic analysis of all the fractionated bladder data yielded an alpha/beta ratio for electrons of 6-7 Gy and, surprisingly, the alpha/beta ratio for neutrons was only slightly higher at about 8 Gy.