We examined the effect of trimidox-induced apoptosis involvement of p53 in the NALM-6 cell line of acute lymphoblastic leukemia. Trimidox has been shown to increase the induction of p53. Phosphorylation of p53 protein at Ser-15 and Ser-20 residues was activated earlier than the obvious increase in p53 expression. Pifithrin-alpha, a p53 inhibitor, significantly prevented trimidox-induced apoptotic characteristics, as detected by nuclear morphological observation and DNA fragmentation. Trimidox-induced apoptosis was enhanced or attenuated by transfection with wild-type or dominant-negative p53 containing expression vectors, respectively. These results indicate that one of the induction mechanisms of apoptosis by trimidox is the mediated augmentation of p53.