A sensitive radiotracer method was used to detect glutamine synthetase activity in fish. The distribution of glutamine synthetase activity using this method was shown to be widespread in organs and tissues of the marine finfish, snapper, Pagrus auratus. The optimum pH and temperature for glutamine synthetase activity and Km Of glutamate from the brain of the snapper are also reported. Interestingly no glutamine synthetase activity was detected in lymphoid organs, indicating, that as in the mammal, the teleostean immune system is likely to rely on glutamine in circulation for energy and nucleotide synthesis. Furthermore circulating glutamine levels fell as fish became stressed or became ill. Implications of these observations on the function of the immune system of fish are discussed.