Fragile sites induced aphidicolin(APC) which acts as a specific inhibitor of DNA polymerase α were studied using human peripheral lymphocyte cultures stimulated with phytohemagglutinin(PHA). It was investigated what concentration and treatment time of APC are the most appropriate to induce chromosomal aberrations and whether there is some difference in the frequencies of fragile sites between early-and late-proliferating cells. In all cases observed, the frequencies of chromosomal breakage at 3p14 were found to be highly remarkable(p<0.001). APC caused chromosomal lesions most effectively when it was treated at a final concentration of 0.2μM during the last 25h of culture. The frequencies of fragile sites were considerably different among cultures undergone the 1st cycle of DNA replication, which, each of them was harvested at different times after PHA stimulation. These results suggest that the cellular specificity in the receptor sites to PHA and the response to PHA stimulation result in the different frequency of fragile sites.