Our previous investigtion revealed a restorative effect of exogenous nucleic acids on the intestinal crypt in mice after abdominal γ-irradiation. In this article, the factors influencing the restorative effect of exogenous RNA on the intestinal crypt in mice post-irradiation were studied. The results showed that: (a)RNAs from different sources all showed the crypt survival enhancement capability.(b) Bell-shaped curves correlating the crypt survival fraction and RNA doses were obtained, with the optimal doses for different routes of administration estimated.(c) Comparing the different routes of RNA administration, the intravenous injection seemed to be the most effective. (d) An exponential relationship between the crypt survival fraction and the post-irradiation time of RNA administration was found. The earlier the administration, the more effective it was. (e) Administration of RNA merely once within 6 h after irradiation, the increases of crypt survival fraction was statistically significant when compared with that of the irradiated control.