Phagocytosis is the predominant defense mechanism of invertebrates. Here we show that phagocytosis by echinoderm bladder amœbocytes and tunicate granular amœbocytes can be enhanced by invertebrate interleukin-1-like molecules. As little as 5 ng/ml of invertebrate interleukin-1 produced a significant stimulation of echinoderm and tunicate amœbocyte phagocytosis. Stimulation of phagocytosis by echinoderm interleukin-1-like molecules was inhibited by antisera to vertebrate interleukin-1. Invertebrate interleukin-1 also acted as an opsonin when preincubated with erythrocytes or yeast. In addition, the cellular mechanisms of invertebrate phagocytosis were studied using pharmacologic agents to inhibit echinoderm amœbocyte phagocytosis. The energy requirements and involvement of cellular cytoskeletal elements in phagocytosis by bladder amœbocytes were similar to those of mammalian macrophages. These results demonstrate a role for interleukin-1 in invertebrate host defense mechanism.