It remains unclear whether the limitations to the therapeutic potential of angiogenic growth factors stem from pharmacokinetic concerns related to inadequate delivery or from a reduced sensitivity of target tissues. Here, we report a novel method using resonance energy transfer to assess the molecular integrity of proteins after local deliv ery. As an example, we labeled fibroblast growth factor-2 with a fluorescent donor and nonfluorescent acceptor pair, tetra methylrhodamine and QSY-7, and demonstrate in an ex vivo bovine carotid artery model that this growth factor is not limited by proteolytic constraints imposed by the tissue. Our data indicate that FGF-2 is unlikely to be degraded within the arterial Wall and suggest that pharmacokinetic limitations alone cannot fully explain the muted response seen thus far in therapeutic angiogenesis. In general, resonance energy transfer may serve as a novel approach to assess the molecular integrity of protein-based therapies in local delivery. (C) 2004 Wiley Periodicals, Inc.