The internal defence system of gastropod molluscs is able to discriminate between self and non‐self. The recognition is carried out principally by both the cellular and humoral components of the haemolymph. Together with freely circulating haemocytes, other defence cells are found scattered throughout the tissue or localized in organs. The present review refers primarily toPlanorbarius corneus, since the defence mechanisms presented by this animal are typical of those of the other gastropods studied.P. corneuspresents two circulating haemocytes: the spreading (SH) and the round haemocytes (RH); in other gastropods only one cell type is described, and this can be considered as a spreading haemocyte. The haemocytes derive both from a haematopoietic organ and from mature circulating haemocytes. The SH show phagocytic properties, adhere to glass, produce agglutinins, bind Con A and contain muramic acid. The RH have non phagocytic properties, do not adhere to glass, form rosettes with sheep red blood cells, are stimulated to proliferate by PHA and present numerous typical markers of vertebrate T lymphocytes. RH are also able to lyse51Cr pre‐labeled K562 target cells in a classical, short‐term, natural cytotoxicity test, and this function is modulated by human recombinant interleukin‐2. Furthermore, SH and RH play a role in the recognition of foreign tissue, the SH are able to encapsulate and phagocytize foreign material, and the RH, with their NK (natural killer)‐like activity, may act like the vertebrate cytotoxic T lymphocytes or NK cells. Thus, it is possible to conclude that RH have characteristics reminiscent of vertebrate T lymphocytes, while SH belong to the category of macrophages. With regards to the humoral component, different factors, such as lysosomal enzymes, lysins and agglutinins or lectins, have been described. InP. corneus, a natural glycoprotein agglutinin has been isolated, whose carbohydrate component contains muramic rather than sialic acid. Moreover, an induced bacterial agglutinin has been purified, although this induction is relatively rare in gastropods. Lysozyme‐like molecules have also been detected and they act like alarm molecules in inflammatory reactions. Taken together, the humoral and cellular investigations, the bacterial clearance studies and the specific responses observed in transplantation experiments are all in favour of the presence of a memory‐type response of short duration. Finally, interrelations appear to exist between the immune and neuroendocrinesystems.ACTH and ß‐endorphin‐immunoreactive molecules have been detected in serum and SH, and these molecules appear to play a physiological role in the process of phagocytosis and in stress response.