To investigate the effect of varying concentrations of PAHs and laboratory holding on hemocyte characteristics, Pacific oysters, Crassostrea gigas, were exposed to varying concentrations of polycyclic aromatic hydrocarbons for 40 days. The circulating hemocyte count (HC) was significantly decreased over the experimental day for all treatments including controls, by day 20; after 10 days, there were no differences among treatments. Similar pattern of decrease was observed over the experimental day in phagocytic index (PI) with relatively constant PI after 20 days. Statistical significance observed only at 10 days. Percent of mobile hemocyte (PMH) also exhibited significant decrease with an association of PAH concentrations without effect of laboratory holding. Lysosomal membrane destabilization (LD) increased continuously over the experimental day with exposed level-dependent differences. Our result indicate that PMH and LD are useful tools for biomarker with sensitivity to toxicant regardless of the effect of laboratory holding, however, HC and PI are significantly influenced by laboratory holding and exposure effect is negligible at the end of the experiment.