Transforming growth factor-b (TGF-b) promotes cancer invasion and metastasis and is therefore a potential drug target for cancer treatment. Fresolimumab, which neutralizes all mammalian active isoforms of TGF-b, was radiolabeled with 89 Zr for PET to analyze TGF-b expression, antibody tumor uptake, and organ distribution. Methods: 89 Zr was conjugated to fresolimumab using the chelator N-succinyldesferrioxamine-B-tetrafluorphenol. 89 Zr-fresolimumab was analyzed for conjugation ratio, aggregation, radiochemical purity, stability, and immunoreactivity. 89 Zr-fresolimumab tumor uptake and organ distribution were assessed using 3 protein doses (10, 50, and 100 mg) and compared with 111 In-IgG in a human TGF-b-transfected Chinese hamster ovary xenograft model, human breast cancer MDA-MB-231 xenograft, and metastatic model. Latent and active TGF-b1 expression was analyzed in tissue homogenates with enzyme-linked immunosorbent assay. Results: 89 Zr was labeled to fresolimumab with high specific activity (.1 GBq/ mg), high yield, and high purity. In vitro validation of 89 Zr-fresolimumab showed a fully preserved immunoreactivity and long (.1 wk) stability in solution and in human serum. In vivo validation showed an 89 Zr-fresolimumab distribution similar to IgG in most organs, except for a higher uptake in the liver in all mice and higher kidney uptake in the 10-mg group. 89 Zr-fresolimumab induced no toxicity in mice; it accumulated in primary tumors and metastases in a manner similar to IgG. Both latent and active TGF-b was detected in tumor homogenates, whereas only latent TGF-b could be detected in liver homogenates. Remarkably high 89 Zr-fresolimumab uptake was seen in sites of tumor ulceration and in scar tissue, processes in which TGF-b is known to be highly active. Conclusion: Fresolimumab tumor uptake and organ distribution can be visualized and quantified with 89 Zr-fresolimumab PET. This technique will be used to guide further clinical development of fresolimumab and could possibly identify patients most likely to benefit.