The acute phase response (APR) is an integrated systemic set of responses to an inflammatory insult in vertebrate animals. The APR has been studied in organisms ranging from fish to man. In this study, an APR is demonstrated in an invertebrate (the purple sea urchin Arbacia punctulata). An experimental inflammatory response (a perceived bacterial challenge) was stimulated by the injection of lipopolysaccharide (LPS). Results show differences in whole protein and glycoprotein profiles between the coelomic fluid of control and stimulated sea urchins as detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The number and proportion of specific cell types in sea urchin coelomic fluids was significantly different in stimulated animals when compared to control animals. Proteolytic enzymes were detected in the stimulated coelomic fluids by both gelatin agar assays and zymography. Proteolytic enzymes were observed in two fractions after size exclusion chromatography of stimulated coelomic fluids. The ability of these proteolytic fractions to react with specific protease substrates and inhibitors attests to their trypsin-like character. Taken together, these results suggest that invertebrate animals employ an APR as part of their immune response to eliminate invaders.