The present study was aimed to investigate the effect of a probiotic, Enterococcus faecium, on the immune responses against infection with the marine fish pathogen Lactococcus garvieae in olive flounder (Paralichthys olivaceus). The immune responses were assessed by lysozyme activity, complement activity, protease activity, and expression of proinflammatory cytokines by RT-PCR. The lysozyme and complement activities were increased between 9 to 15 and 9 to 13 days, respectively, and antiprotease activity was slightly elevated after 5 days of probiotic treatment. The TNF-alpha and IL-1beta expressions were observed from kidney and spleen. The results of this study reveal that E. faecium induces immune-responsible materials and protects olive flounder from lactococcosis.