alpha 2-Antiplasmin (alpha 2-AP) is a major fibrinolysis inhibitor, whose complete, congenital absence has been found to be associated with a distinct hemorrhagic diathesis. We studied a 15-yr-old male with a hemorrhagic diathesis after trauma from early childhood on. This bleeding tendency was associated with a minimal alpha 2-AP level recorded functionally in the immediate plasmin inhibition test: less than or equal to 4% of normal. However, a normal plasma concentration of alpha 2-AP antigen (83%) was found. His sister (5 yr old) showed similar results (2 and 92%). In their family, eight heterozygotes could be identified by half-normal activity results and normal antigen concentrations. The inheritance pattern is autosomal recessive. On analysis, the alpha 2-AP of the propositus was homogeneous in all respects tested, suggesting a homozygous defect. We designated the abnormal alpha 2-AP as alpha 2-AP Enschede. alpha 2-AP Enschede showed the following characteristics: (a) complete immunological identity with normal alpha 2-AP; (b) normal molecular weight (sodium dodecyl sulfate-polyacrylamide gel electrophoresis); (c) normal alpha-electrophoretic mobility; (d) presence in plasma of both molecular forms excluding an excessive conversion to the less reactive non-plasminogen-binding form; (e) quantitatively normal binding to lys-plasminogen and to immobilized plasminogen kringle 1-3; and (f) normal Factor XIII-mediated binding to fibrin. Functional abnormalities were found in: (i) no inhibition of amidolytic activities of plasmin and trypsin, even on prolonged incubation; (ii) no formation of plasmin-antiplasmin complexes in plasma with plasmin added in excess; and (iii) no inhibition of fibrinolysis by fibrin-bound alpha 2-AP. In the heterozygotes, the presence of abnormal alpha 2-AP did not interfere with several functions of the residual normal alpha 2-AP. One-dimensional peptide mapping showed an abnormal pattern of papain digestion. We conclude that in this family, abnormal antiplasmin molecules, defective in plasmin inhibition but with normal plasminogen-binding properties, have been inherited. The residual plasminogen-binding properties do not protect against a hemorrhagic diathesis.