A method for the specific determination of a form of alpha 2-antiplasmin which does not interact with the lysine-binding sites in plasminogen (NPB-AP) has been elaborated. Basically, the method is an electroimmnoassay utilizing an intermediate gel, which adsorbs the plasminogen-binding form of alpha 2-antiplasmin (PB-AP). The plasma concentration of NPB-AP in healthy individuals was determined as 0.40 mumol/1 +/- 0.14 (SD), constituting 35% +/- 11 (SD) of the total plasma alpha 2-antiplasmin concentration. During extensive plasmapheresis of two pregnant severely D-immunized women the NPB-AP form decreased significantly, while the PB-AP form increased, thus maintaining the total alpha 2-antiplasmin at an almost constant level. The increased biosynthesis rate of alpha 2-antiplasmin during the extensive plasmapheresis is thus accounted for by PB-AP indicating this form to be the one primarily synthesized and that the non-binding form is formed in plasma from PB-AP secondarily.