To determine the role of Src homology protein tyrosine phosphatase (SHP-1) in the ionizing radiation-induced stress response, we analyzed the apoptotic response and cell cycle function in irradiated spleen cells of motheaten (me/me) mice. The defect in me/me mice has been attributed to mutations of the Hcph gene, which encodes SHP-1. Homozygotes develop severe systemic autoimmune and inflammatory disease, whereas heterozygotes live longer and develop hematopoietic and lymphoid malignance. Spleen cells from C57BL/6 (B6)-me/me and B6-+/+ controls were analyzed after γ-irradiation from a 137Cs source. B6-me/me cells were significantly more resistant than B6-+/+ cells to γ-irradiation-induced apoptosis exhibiting a higher LD50. The defective apoptosis response of the B6-me/me cells was exhibited by T and B cells and macrophages. Of the Bcl-2 family members analyzed, a significant difference was observed in the transcription of Bax mRNA, which was up-regulated early after irradiation in B6-+/+ cells, but not B6-me/me cells. Analysis of 3,3′-dihexyloxacarbocyanine iodide revealed resistance to the γ-irradiation-induced mitochondrial transmembrane permeability transition in the B6-me/me cells. The blocking of the cell cycle in the G0/G1 phase characteristic of the irradiated B6-+/+ cells was not observed in the B6-me/me cells. There was decreased phosphorylation of p38 mitogen-activated protein kinase and increased phosphorylatlon of p53 from spleen cell lysates of irradiated B6-me/me mice compared with wild-type mice. These data suggest that SHP-1 plays an important role in regulation of apoptosis and cell cycle arrest after a γ-irradiation-induced stress response.