Plasma antithrombin III (AT III), alpha 1-antitrypsin (alpha 1-AT) and alpha 1-macroglobulin (alpha 1-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 less than 0.005), cholesterolaemia (r = -0.651, p less than 0.005) as well as with urinary protein (r = -0.531, p less than 0.05). The alpha 1-AT concentration was decreased (p less than 0.01) and correlated with serum albumin level (r = +0.643, p less than 0.01). The alpha 1-M concentration significantly increased (p less than 0.001). A negative correlation of the alpha 1-M level with serum albumin concentration (r = -0.561, p less than 0.025) and a positive correlation with the cholesterol concentration (r = +0.819, p less than 0.001) was found. AT III and alpha 1-M were inversely correlated (r = -0.706, p less than 0.005). It is considered that in NS in adults, the increase of alpha 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.