This article describes experiments performed to examine the possible effect of interaction between ionizing radiation and magnetic resonance (MR) on damage to normal tissue. Eight-week-old ICR male mice were irradiated (cobalt-60 radiation) with 5, 6, or 7 Gy given either alone or followed by MR imaging. Other groups received fractionated doses of 6 Gy (3 Gy + 3 Gy) or 7 Gy (3 Gy + 4 Gy) either with or without subsequent MR imaging. Ten days after exposure, spleens were assayed for endogenous spleen colonies. The number of spleen colonies was lower at higher radiation doses, and fractionation of the dose resulted in an increase in colony number compared with a comparable dose in a single exposure. No difference was seen, however, between comparable radiation groups that were or were not subjected to MR imaging. In addition to the spleen colony assay, body weights and wet weights for spleen, thymus, and testes were obtained, since these suffer weight loss in proportion to radiation dose. As with the spleen colony assay, no significant effect of MR imaging was observed. These results indicate that for the normal tissues studied, MR imaging neither increases radiation damage nor inhibits repair between fractions.