Immune response is dependent on synthesis and secretion of various proteins. Hence, in vivo determination of protein synthesis rate in leukocytes may reflect the functional activity of these cells under conditions of immune deficiency such as aging. Because (i) insulin stimulates protein turnover in various tissues and (ii) elderly people often suffer from a decrease in insulin sensitivity, we sought to determine the effect of physiological hyperinsulinemia (0.7mU/kg fat free mass/min) on protein synthesis rate in peripheral blood mononuclear cells (PBMCs) and polymorphonuclear neutrophils (PMNs) from 8 adult (24+/-6yr) and 9 elderly (69+/-4yr) healthy subjects under strict euglycemia (0.9g/l). The body mass index was significantly higher in elderly volunteers compared to the younger ones (P<0.05) whereas insulin sensitivity was lower as shown by either the M value (P<0.05) or the glucose disposal rate (GDR)/insulinemia ratio (P<0.05). The density of the insulin receptor at PBMCs and PMNs surfaces was not altered by either age or insulin infusion. In comparison with the baseline values, insulin highly stimulated the fractional synthesis rate (FSR) of mixed PBMCs (4.12+/-0.56%/d vs. 6.94+/-0.70%/d, P<0.05) and PMNs (1.05+/-0.28%/d vs. 2.44+/-0.41%/d, P<0.05) proteins. No age effect was observed either during the baseline (PBMCs: 4.31+/-0.30%/d, PMNs: 1.55+/-0.28%/d) period or after insulin infusion (PBMCs: 6.86+/-0.69%/d, PMNs: 2.76+/-0.47%/d). We conclude that insulin is able to stimulate protein synthesis in leukocyte populations independently of the age of the subjects.