An 81-year-old male with a mild life-long bleeding history and an alpha 2-antiplasmin (alpha 2-AP) plasma level of 55% biological activity and 41% antigen activity (normal range 80-140%) was studied. The ratio of plasminogen binding (PB):non-plasminogen binding (NPB) alpha 2-AP assayed by modified crossed immunoelectrophoresis (CIE) was 7.3/2.7 (controls 6.3 +/- 0.49 SD/3.7 +/- 0.49 SD). The patient's alpha 2-AP showed decreased affinity for fibrin, i.e. 8.3% versus 32.4% of normal control alpha 2-AP associated with fibrin during clotting of plasma. A metabolic study performed with human purified 125I-alpha 2-AP (PB/NPB 7.7/2.3) showed a plasma radioactivity disappearance half-life of 72.9 h (n 60.1 +/- 5.3 h) with a normal fractional catabolic rate and a reduced absolute catabolic (synthetic) rate of 0.70 mg/kg/day (n 2.10 +/- 0.60 mg/kg/day). The exchange between the central and third compartment was increased. The increased alpha 2-AP PB form and the increased plasma radioactivity disappearance half-life are suggestive of a slower conversion of the PB form into the NPB form and/or slower degradation of the PB form. The bleeding tendency in this patient could be explained by decreased synthesis of alpha 2-AP and decreased binding to fibrin.