In this study, we describe a novel function of the p34(SEI-1) protein, which is both an oncogenic protein and a positive regulator of the cell cycle. The p34(SEI-1) protein was found to inhibit doxorubicin-induced senescence. We investigated the molecular mechanisms of the inhibitory effect of p34(SEI-1) on senescence. First, we found that the activation of protein kinase C-delta (PKC-delta), which is cleaved into a 38 kDa active form from a 78 kDa pro-form, induced after doxorubicin treatment, was inhibited by p34(SEI-1). Furthermore, p34(SEI-1) induced the ubiquitination of PKC-delta. Yet, there is no interaction between p34(SEI-1) and PKC-delta. We also found that the phosphorylation of c-Jun-NH(2)-kinase 1 (JNK1) induced after doxorubicin treatment was suppressed by p34(SEI-1), but not in JNK2. Consistently, pharmacologic or genetic inactivation of either PKC-delta or JNK1 was found to inhibit doxorubicin-induced senescence. In addition, the genetic inactivation of PKC-delta by PKC-delta small interfering RNA resulted in an inhibition of JNK1 activation, but PKC-delta expression was not inactivated by JNK1 small interfering RNA, implying that the activation of JNK1 could be dependently induced by PKC-delta. Therefore, p34(SEI-1) inhibits senescence by inducing PKC-delta ubiquitination and preventing PKC-delta-dependent phosphorylation of JNK1.