Plasma antithrombin III (AT III), α1-antitrypsin (α1-AT) and α1-macroglobulin (α1-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 1-AT concentration was decreased (p 1-M concentration significantly increased (p 1-M level with serum albumin concentration (r= -0.561, p 1-M were inversely correlated (r= –0.706, p 1-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.