This study was designed to investigate the expression of aminopeptidase N (APN)/CD13 on intraembryonic AGM stromal cells, and the change of its enzymatic activity after irradiation injury. The expression of APN/CD13 on AGM stromal cells was assayed by RT-PCR and immunohistochemistry. After the stromal cells in AGM region were irradiated with 8.0 Gy of (60)Co gamma-rays, APN/CD13 enzymatic activity was measured by spectrophotometer at different time points. The result showed that AGM stromal cells strongly expressed APN/CD13. The enzymatic activity of APN/CD13 decreased temporarily after irradiation injury, then increased to higher level 4 h after irradiation, and it returned to the pre-irradiation level 24 to 48 h after the irradiation. The enzymatic activity of APN/CD13 was temporarily enhanced after irradiation injury, which might be one of the compensatory mechanisms that promote the hematopoietic recovery after irradiation.