The Mollusca is one of the largest of invertebrate phyla. Two major classes, the Gastropoda and the Pelecypoda, have been the subject of numerous studies on immunity and neoplasia. Investigations of immunity have dealt with cellular and humoral aspects, phagocytosis and encapsulation, and rejection of foreign tissue grafts. Work on humoral responses has focused on lysozyme, the hemagglutinins (especially in the oyster), and the clearance of certain antigens. Neoplasms in these animals resemble certain cancers in vertebrates, but it is not clear whether a relationship exists between the invertebrate immune system and the development of neoplasia. Studies of immunity and neoplasia in invertebrates may reveal how these two phenomena of living systems have evolved.