The CDK inhibitor, p21, exhibits an antiapoptotic or proapoptotic effect, in addition to its anti-proliferative effect, depending on the conditions. To define the apoptosis-regulatory function of p21, we constructed cells that stably express C-terminal deletion mutants of p21 (full length 164aa), 1-157, 1-147 or 1-128, and evaluated the apoptotic response of these cells. The AnnexinV positive cell fraction after gamma-irradiation did not increase in cells expressing 1-157. Consistently, an increase of caspase3 activity or the active form of caspase3 was not observed in cells expressing 1-157, but was prominent in cells expressing 1-128 and 1-147. Further, the activity of caspase9 was suppressed in gamma-irradiated cells expressing 1-157. The antiapoptotic effect of 1-157 was weaker in Fas-induced apoptosis. Our data indicate that the 1-157 region of p21 inhibits apoptosis caused by gamma-irradiation by reducing the activity of caspase9 and caspase3, and the 148-157 region is critical for its inhibiting activity.