Taking into account the great sensitivity of the intestinal mucosa to X irradiation we aimed at investigating the radioprotection afforded by the S-2-(3-aminopropylamino) ethylphosphorothioic acid (WR2721) on the mice rectum exposed to a single whole body X ray dose of 7 Gy and sacrificed at 1, 3, 6 and 10 days after irradiation. Comparing the data of protected group with those of the unprotected one the following features were observed in the first one: statistically significant increase of both crypt cells per crypt (CrC/Cr) at 1, 3, 6 and 10 days after irradiation and of mitotic figures per crypt (MF/Cr) at 3, 6 and 10 days post exposure. In this way it became clear that these morphometric parameters allowed the estimation of WR2721 rectum protection through the crypt cellularity and epithelial cell kinetics