Summary: Rabbits were immunized with the enzyme Cypridina luciferase and the antiluciferase activity in spleen and lymph node extracts was studied chromatographically. The extracts were separated into major fractions by differential centrifugation and into subfractions by gel exclusion chromatography. Only two types of soluble antiluciferase activity were found. Based on elution from Sephadex G 200 columns, the first type of activity appeared to be due to immunoglobulin-G (IgG). The second type possessed a molecular weight between 50,000 and 70,000 and resembled Porter's fragments I and II. Significant amounts of luciferase were associated with the high molecular weight fraction of lymph nodes.