OBJECTIVE This study was aimed to gain an insight into the molecular basis of S-phase arrest associated apoptosis. METHODS The molecular expressions of Rb, cyclin A, cyclin E, CDK2 and CDC2 involved in S-phase progression were investigated by westernblot in L-2 and Br1-3pr-1 cells. Furthermore, their associated kinase activities were assayed by histone, phosphorylation. RESULTS The results showed that ofter PALA treatment, the expression of cyclin A increased and the expression of Bcl-2 decreased in apoptosising L-2 cell as compared with those in unapoptosising Br1-3pr-1 cell, but there were no significant differences in other molecular expressions and associated kinase activities between the above-mentioned L-2 cell and Br1-3pr-1 cell arrested at S phase after PALA treatment. CONCLUSION Cyclin A is a most potential candidate for S phase checkpoint element, and it may be able to induce the S phase arrest associated apoptosis through the Bcl-2 pathway.