We have studied the plasminogen-binding and non-binding molecular forms of α2-antiplasmin (α2-AP), the major inhibitor of plasmin, using a modification of the crossed immunoelectrophoresis technique (mCIE). The previous technique using addition of Lys-plasminogen to the gel of the first dimension electrophoresis was simplified using commercially available monoclonal antibodies (MAb) against the COOH-terminal part of α2-AP, which harbours the plasminogen binding site. Indeed, the addition of MAb resulted in a mobility reduction of the plasminogen binding (PB) form of α2-AP, with an apparent Kd of 12.5 nM, whereas the non-plasminogen binding form (NPB-α2-AP) in plasma retained its normal mobility. The method with MAb was applied to study the molecular forms of α2-AP in plasma and serum of healthy individuals and of patients with various diseases as well as in other body fluids, such as ascites or synovial fluid. Upon investigation the conversion of PB-α2-AP into NPB-α2-AP in vitro, however we found evidence for a difference in specificity between the method using Lys-plasminogen and MAb. Though in-vitro-formed NPB-α2-AP loses its plasminogen binding, its MAb binding remains intact. This is in contrast to in vivo circulating NPB-α2-AP which has lost both properties. In selected samples of the conditions mentioned above we found no evidence for significant occurrence of the MAb binding NPB-α2-AP in vivo. We suggest therefore that the conversion in vivo and in vitro have different mechanisms and that the MAb is a useful tool for study of patient samples and for the further investigations into the mechanism of conversion.