Many occasions arise in fish culture in which it is desirable to enhance the immune response. These include strengthening the normal immune response in order to enhance protection and reduce immunosuppressive conditions. Several categories of immunostimulators can be classified by their origin and mode of action: (1) bacteria and bacterial products; (2) complex carbohydrates; (3) vaccines ('paramunity inducer'); (4) immunoenhancing drugs; (5) nutritional factors; (6) animal extracts; (7) cytokines; (8) lectins, plant extracts. In this paper, the importance of many immunostimulatory substances and their effects on fish immune systems are discussed. Two main procedures for evaluating the efficiency of an immunostimulant are: (a) 'in vivo', e.g. protection tests against fish pathogens; (b) 'in vitro', e.g. measurement of the efficiency of cellular and humoral immune mechanisms; the efficiency of these different evaluation methods is discussed. Attention is focused on the lymphocyte proliferation test as an adequate method for providing a correct evaluation of cellular immune conditions which can be adapted together with the more commonly used parameters, such as phagocytosis and respiratory burst. The use of immunostimulants in the diets of marine fish and the evaluation of their effect on the immune system of fish are also discussed.
