Glutamine is the most abundant amino acid in the mammalian body and is essential for many homeostatic functions. It is a precursor for nucleotide biosynthesis and is a principal fuel for metabolically active cells, particularly those of the immune system. In the early 1980's it was established that glutamine was an absolute requirement for proliferating mammalian lymphocytes. Current hypotheses suggest that lowered peripheral blood glutamine may be associated with a reduced capacity of the immune system. It has now been demonstrated that a lowered concentration of circulating glutamine is a good indicator of stress and overtraining in man. Developing from the mammalian model, we have been investigating the role of glutamine in fish with the aim of minimising the incidence of disease in farmed situations.The commercially important marine fish, snapper (Pagrus auratus) was used in these studies. Preliminary research indicated that the major lymphatic organs have minimal activities of the enzyme, glutamine synthetase and thus may not be able to synthesise adequate amounts of glutamine. These results suggest that the immune system of snapper may have a similar dependence on circulating levels as in mammals. Snapper lymphocytes were used to perform mitogen proliferation assays over a range of glutamine concentrations and the activity of glutamine synthetase was determined. The results indicate that snapper lymphocytes respond differently to their mammalian counterparts.