Lysozymes are shown to be involved in broad battery of defense mechanisms such as bateriolysis, opsonization, anti-viral and anti-neoplatic activity in both vertebrates and invertebrates. Previous experiments in our laboratory showed that Perkinsus marinus infection in oysters was positively correlated with temperatures and salinities. In contrast, plasma lysozyme activity in oysters was negatively correlated with temperatures and salinities. To elucidate the role of lysozyme(s) in defense against the parasite, Perkinsus marinus, in oysters, we have purified lysozyme(s) from oyster plasma. Lysozyme was partially purified using ion-exchange chromatography using carboxymethyl cellulose (CMC) as adsorbant. The buffer systems consisted of 0.01M sodium acetate with NaCl (0-0.8M). Based on the lysozyme activity in eluted fractions, lysozyme(s) was purified nearly 700 folds. Experiments are underway to characterize purified lysozyme(s) from plasma and examine its effect on P. marinus viability and proliferation.