We immunohistochemically examined the effect of gamma-radiation on immature mouse ovarian follicles. Mice were Jirradiated with a dose of LD80 (8.3 Gy) in KAERI. At 0 h, 6 h, 12 h, 1 d, 2 d, 4 d and 8 d postirradiation, the ovaries were excised and fixed in neutral buffered formalin. We performed immunohistochemistry for protein retinoblastoma (pRb), terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end-labeling (TUNEL) and routine hematoxylin-eosin staining in the largest cross sections. Radiation-induced follicular degeneration increased before 6 h, and most irradiated ovarian follicles became acutely atretic. The immunohistochemical staining for pRb was strong in the nuclei of granulosa cells of normal follicles and weak in atretic ones which were, conversely, strong for TUNEL staining. It was shown that pRb expression became lower with the degeneration of the ovarian follicles, which was inhibited by gamma-radiation. In the present study, pRb immunohistochemistry was proven to be a useful tool for the identification of follicular status.