The microcolony assay technique was used to test the validity of summing equivalent doses per fraction of 14 MeV neutrons and .gamma. rays for mouse intestinal damage. For a 4-daily fraction schedule, in which the 1st 1 or 2 fractions are given as neutrons and the remainder as .gamma. rays, combined dose fractions calculated from a 4-fraction schedule of either radiation type alone produce the same level of damage within the limits of accuracy of the experiment.