The activity and location of phendoxidase ( PO) in lymphatic organ of Penaeus chinensis were studied by enzyme cyto-chemical technique of electronic microscope. The results showed: 1. Few PO positive reaction in lymphat ic organ of health individual was observed. 2. In the sick, the activity of PO in lymphatic organ appeared in different degree. Observed by electronic microscope, the multiplying and expanding of smooth endoplasmic reticulum in the endothelial cell of lymphat ic tubules were induced by virus invading. Meanwhile, the activity of PO appeared; Many PO positive granular and myelinef igure existed in the cells of lymphatic tubule wall; In the exterior lymphatic tubule, the quantity of virus reduced obviously. PO positive granular in the hemal sinuses can be found secreted from tubule cell. It is inferred: ( a ) PO can destroy virus. ( b) Lymphatic tubules have the effects of filtering haemo- lymph and secreting PO granular.