Plasma antithrombin III (AT III), α<sub>1</sub>-antitrypsin (α<sub>1</sub>-AT) and α<sub>1</sub>-macroglobulin (α<sub>1</sub>-M) concentrations were measured in 17 cases of the nephrotic syndrome (NS) in adults. The mean plasma level of AT III was normal. The AT III concentration was correlated with albuminaemia (r=+0.718, p < 0.005), cholesterolaemia (r= – 0.651, p < 0.005) as well as with urinary protein (r = – 0.531, p < 0.05). The α<sub>1</sub>-AT concentration was decreased (p < 0.01) and correlated with serum albumin level (r = + 0.643, p < 0.01). The α<sub>1</sub>-M concentration significantly increased (p < 0.001). A negative correlation of the α<sub>1</sub>-M level with serum albumin concentration (r= -0.561, p < 0.025) and a positive correlation with the cholesterol concentration (r= +0.819, p < 0.001) was found. AT III and α<sub>1</sub>-M were inversely correlated (r= –0.706, p < 0.005). It is considered that in NS in adults, the increase of α<sub>1</sub>-M is not compensatory for AT III, and there is no objection to the use of heparin in thrombosis prophylaxis with the exception of a few cases with very low AT III levels.