Abstract Plasma β-thromboglobulin (βTG) was measured by radioimmunoassay in 219 healthy subjects age range (12–103), arbitrarily dividied into 3 groups: 127 young, 35 middle aged and 57 old. In 73 of the subjects, plasma platelet factor 4 (PF4) level was also determined by RIA. Plasma βTG (mean 33.9ng/ml), range 3–95) and PF4 (mean 15.2 ng/ml, range 6.62–50) levels increased with age with a significant difference between the mean plasma level of the 3 groups. In addition there was a significant sex difference in old females compared to old males. A significant correlation between the 2 proteins was found in each of the 3 groups of subjects and in the whole group suggesting that both proteins are released in vivo from the same pool and presumably at the same rate. Our results indicate that in vivo platelet activation and “release reaction” is significantly enhanced in old and middle aged subjects compared to young suggesting abnormal hemostatic mechanisms in these individuals. This enhanced platelet activity may reflect a prethrombotic state.