This study was performed to investigate the immunological side effects of chloramphenicol (CAP) on olive flounder, Paralichthys olivaceus. To investigate immunological effects on olive flounder, we determined the changes of chemiluminescence (CL) response of flounder kidney-derived leucocyte after the treatment of CAP in vivo and in vitro. The CL activity was significantly decreased in a dose-dependent manner during the treatment of CAP in vitro. Similarly, a dose-dependent reduction of CL response, although not significant, were observed during the treatment of CAP in vivo. The results suggest that CAP reduced the function of flounder phagocytosis in vivo and in vitro, indicating the immunosuppressive ability of CAP