A partially purified protein (the SR fraction) of porcine and human origin has been extensively characterized as Follicular Regulatory Protein (FRP). In the current study, 1A8D5, one of several monoclonal antibodies raised against FRP, was used to further purify the protein. The monoclonal antibody cross-reacted only with porcine plasminogen, a key fibrinolytic proenzyme. A commercial polyclonal antibody for human plasminogen confirmed the relationship between plasminogen and bands of the SR fraction of the porcine follicular fluid. Sequencing of the N-terminal amino acids (54 kd) of the SR fraction indicated that it shared 100% identity with the short form of porcine plasminogen chain A and 93% identity to human plasminogen. Moreover, we demonstrated that this purified protein from human follicular fluid inhibited aromatase activity of granulosa cells, a key biological property of FRP. Given that plasminogen possesses most of the proposed properties of the protein termed FRP, we conclude that FRP is likely plasminogen itself or a plasminogen-related protein and not a novel protein.