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It has been suggested that increased susceptibility to infection sometimes seen following surgery might be related to levels of circulating glutamine. However, previous studies have not investigated the degree of immune cell activation in relation to availability of glutamine after surgery. In seven patients plasma glutamine concentration decreased by about 50% immediately after major vascular surgery, and took 5 days to recover to normal levels. Although plasma glutamate concentration was inversely related to glutamine, glutamate levels were some three times lower. The circulating numbers of platelets, neutrophils, monocytes and lymphocytes (specifically CD4+ cells) were significantly related to both the concentration of glutamine and glutamate irrespective of time. However, there was no evidence of cell activation as indicated by expression of activation markers (CD69, CD25, CD71 or HLA-DR) on lymphocytes. Creatine kinase activity significantly increased after operation, indicating the presence of substantial post-operative trauma. Transferrin, iron and magnesium were the other biochemical parameters which significantly decreased after surgery, while albumin levels suggested that blood transfusions may have contributed to the changes seen.