Phagocytosis of erythrocytes by phagocytes from the sea urchin Strongylocentrotus nudus can occur in vitro, and is enhanced by opsonization with the coelomic fluid. This opsonic activity of coelomic fluid can be elevated over a 5-day period by injecting erythrocytes into the coelom. Phagocytes produce hydrogen peroxide during both resting and stimulated states. This result on hydrogen peroxide production is the first to be observed in echinoderms. During the stimulated state, phagocytes produce more hydrogen peroxide than resting phagocytes. However, hydrogen peroxide production by phagocytes is not affected by opsonic activity of the coelomic fluid. Phagocytes share similar functional properties with vertebrate macrophages and granulocytes.