&NA;The aim of this study was to characterize soluble fibrin(ogen) species in human, arterial, in‐vivo‐formed thrombi, using the immunoblotting technique. Specimens were collected via intra‐arterial catheters in six patients scheduled for catheter‐directed thrombolysis. Unreduced and reduced samples of the supernatants from the arterial thrombi‐derived specimens were electrophoresed on polyacrylamide gels and immunoblotted, using specific mono‐ and polyclonal anti‐fibrin(ogen) antibodies. The reduced samples disclosed substantial amounts of high molecular weight material, consistent with &agr;‐chain polymers and &ggr;&ggr;‐dimers, as well as lower molecular weight material, such as &agr;‐, &bgr;‐ and &ggr;‐chains. No fibrinogen with intact fibrinopeptide A was detectable, and des‐AABB fibrin represented a major fibrin derivative in the soluble part of the arterial thrombi. The &agr;‐chains were C‐terminally degraded, most of them distal to position 259. In conclusion, we have demonstrated the presence of cross‐linked fibrin derivatives in soluble, arterial thrombus‐related material, without signs of fibrinogen‐fibrin hybrids. The fibrin derivatives were C‐terminally degraded, thus representing X‐oligomeric material, most probably originating from plasmin degradation of insoluble thrombus fibrin. The present study supports the hypothesis of a dynamic equilibrium between clotting and lysis in thrombi.