The levels of plasminogen, α1-antitrypsin, α2-macroglobulin and the platelet count were measured in 511 blood donors. The mean level of α1-antitrypsin was significantly lower in men of group B compared with that of group O. No other differences between the blood groups reached statistical significance. Women had higher mean levels of α1-antitrypsin and α2-macroglobulin and a higher platelet count than men. The levels of plasminogen and α1-antitrypsin were significantly higher in women using oral contraceptives compared with those who were not. The level of α2-macroglobulin fell with age until the 60–64 year age-group in men and the 40–49 year age-group in women. A positive correlation existed between the α1-antitrypsin and the α2-macroglobulin level and between the platelet count and the plasminogen level.