BACKGROUND In this study we evaluated the effect of medroxyprogesterone acetate (MPA) and its major ingredients on protection of the hematopoietic organs against radiation damage. METHODS One group of mice was given saline as placebo and the other groups were given MPA. Mice were injected with MPA (10 mg/kg) or saline 10 days before or after a single 8 Gy whole body cobalt irradiation. On day 14 the mice were sacrificed and their bone marrow transplanted to recipient mice. Ten days after the transplantation, spleen colony formation was investigated in mice. RESULTS Administration of MPA with irradiation increased the formation of the spleen colony. Statistically significant enhancement of the spleen colony formation was found in mice treated with MPA repeatedly, as compared with those treated with placebo (P < 0.001). No significant difference in Spleen Colony Forming Unit (CFU-S) numbers was observed between pre-and post-radiotherapy administration of MPA (P = 0.216). CONCLUSIONS It is an important observation that no significant difference was observed in CFU-S numbers between pre- and post-irradiation administration of MPA.