A significant decrease of plasma antithrombin III (AT III) levels, measured with four different biological and immunological methods, was found in 8 of 11 members of a Sicilian family (family DM) and in 3 of 14 members of a northern Italian family (family A). Different behaviour after oral anticoagulant treatment with acenocoumarin was seen in 2 long-term treated subjects. The propositus of family DM, who had had a long history of recurrent thrombosis, did not show any increase of AT III levels. A significant increase was, on the contrary, observed in the propositus of family A, who had suffered a recent thrombosis in a branch of the inferior mesenteric vein.