Using human lymphoytes stimulated with phytohemagglutinin (PHA), we investigated whether there is some difference between early- and late proliferating cells in the frequencies of fragile site induced by Aphidicolin (APC) which acts as a specific inhibitor of DNA polymerase. It was traced what concentration and treatment time of APC is the most appropriate to induce chromosomal aberrations in lymphocytes. We found that APC caused chromosomal lesion the most effectively when it was treated at 0.2μM for 25 hrs prior to harvest. In all cases observed, the frequencies of chromosomal breakage at 3p14 were found to be highly remarkable (p<0.01), and the frequencies of fragile site were considerably different among cultures undergone the 1st cycle of DNA replication, which each of them was harvested at different time aater PHA stimulation. These findings suggest that the cellular specificity in the receptor sites to PHA and the response to PHA stimulation may result in the different frequency of fragile sites.