This study was conducted to reveal new proteins involved in acute myeloid leukemia (AML) cell apoptosis.Using camptothecin analog NSC606985- induced leukemic U937 cell apoptosis as a model, this study performed a differential proteomic analysis during apoptosis induction. The significantly modulated protein was underwent further investigation in the apoptotic process.We found that beta-actin protein presented two different spots on the two-dimensional electrophoresis (2-DE) map, which shared similar molecular weight and different pI. Those two spots demonstrated contrary changes (disappeared on the basic-end and increased on the acid-end spot) during apoptosis induction, although the total level of beta-actin kept constant. This observation was further confirmed by immunoblot analysis on 2-DE gel. When NSC606985-treated cell lysate was incubated with alkaline phosphotase, beta-actin on the basic-end spot was restored, indicating increased phosphorylation of beta-actin during NSC606985- induced apoptosis. Moreover, the polymerization of actin also decreased after NSC606985 treatment. The increased beta-actin phosphorylation and decreased actin polymerization was antagonized by pre-treatment of rottlerin, a specific protein kinase C-delta (PKC delta) inhibitor.All these results indicate that beta-actin was phosphorylated during apoptosis induction, which was mediated by activated PKC delta.