This study examined the role of hard clam (Meretrix lusoria) hemocytes in the immune defense mechanism, and sought to establish the hemato-immunological parameters. In cytochemical staining, two types of granulocytes (large and small eosinophilic granulocytes) and the hyalinocyte exhibited the periodic acid-Schiff positive reaction for polysaccharides, and all four types of hemocytes contained no granule with a Sudan black B positive signal. Acid phosphatase, β-glucuronidase, and naphthol AS-D chloroacetate esterase activity was found in the granulocytes, while only α-naphthyl butyrate esterase activity was found in hyalinocytes. 98% of the hemocytes tested positive for peroxidase activity. Approximately 37% of the cells exhibited a positive reaction for phenoloxidase, but which type of hemocyte the enzyme was located in was uncertain. In immunological assays, pinocytosis, phagocytosis and superoxide anion generation of the hemocytes were observed. The granulocytes have more activity than hyalinocytes in the uptake of neutral red. While the granulocytes and hyalinocytes were able to phagocytose fluorescent latex beads and zymosan particles, small eosinophilic granulocyte had a stronger phagocytic ability.