The formation of a complex between the fibrin fragments DD and E was studied by crossed immunoelectrophoresis using antibodies against human fibrinogen. The complex formation was seen by a common electrophoretic migration of the DD-fragment and part of the E-fragments. This effect was abolished by a further incubation with plasmin of the preparation containing the (DD) E-complex. This also led to an anodal shift in migration of the E-fragment indicating a transfer from E1 to E3.