||Acute lymphoblastic leukemia (ALL) is the most common pediatric malignancy. Although patients with ALL have benefited greatly from dramatic advances in chemotherapy, a substantial number of patients eventually fail to respond to treatment, and their prognosis is poor. Cytotoxic chemotherapeutic agents kill leukemic cells by inducing apoptosis. The process of apoptosis is regulated by the relative levels of pro- and anti-apoptotic proteins. However, changes in levels of pro- and anti-apoptotic proteins in human leukemia samples in response to therapy remain largely unknown. In childhood leukemia the apoptosis of blast cells in peripheral blood before and during treatment is of great interest. In this study, leukemic blast cells were collected from 16 children newly diagnosed with leukemia and from 3 patients who relapsed. Blasts were harvested from the peripheral blood at 0, 1, 6, 24, and 48 hours following the onset of chemotherapy. Both pro- and anti-apoptotic proteins including p53, p21, MDM-2, BCL-2, BCL-Xl, BCL-Xs, and BAX were analyzed by Western Blot. An ex vivo culture system was developed to assess baseline apoptosis and increased apoptosis associated with treatment. Results demonstrated variability in apoptosis among circulating blasts at diagnosis and after 24 hours of chemotherapy. Using the ex vivo culture system, apoptotic patterns could be divided into low and high response groups based on whether a 30% increase in apoptosis was observed in response to chemptherapy. Relapsed cases failed to show an increase in apoptosis with treatment. Among 14 patients who received drugs that induce apoptosis via a p53-dependent pathway, 7 patients showed an increase in p53 protein levels. There was a correlation between apoptosis ex vivo culture and p53 levels in most cases. The early apoptosis following chemotherapy in culture correlated with changes in p53 protein expression. Diverse patterns of changes in BCL-2 family protein expression occurred in response to treatment. Although all but two patients showed a significant decrease in circulating tumor cells by 24 hours of therapy, the most profound changes in protein expression were observed between 24 and 48 hours. This suggests that the levels of BCL-2 family proteins examined here may not be involved in the early apoptotic response following chemotherapy. This study revealed profiles of kinetic changes of cellular pro- and anti-apoptotic proteins in leukemic cells in response to chemotherapy. Apoptotic response to anticancer drugs is important in determining drug sensitivity and resistance. These results will lead to a better understanding of the apoptotic mechanism of drug resistance to chemotherapy in leukemia.