The hypothesis tested during Phase I was that an ideal filtration marker can be labeled with a metal that when neutron activated generates a strong photon signal suitable for the measurement of glomerular filtration rate (GFR). We successfully proved our hypothesis and demonstrated concept feasibility. The results of our Phase I study have identified three promising agents. Our Phase II research will expand on this work by directly comparing our novel method to traditional methods of measuring GFR (sodium iothalanate (I-125) and 51-Cr-EDTA) using an in vivo rabbit model. In addition, Phase II will evaluate the biodistribution of each agent and will initiate preclinical toxicology studies necessary to carry the project forward to Phase lIl. Our goal is to identify an ideal filtration marker that can be labeled with two different stable isotopes. As a result, we can provide a non-radioactive GFR assay kit that is safe, easy-to-use, accurate, and cost-effective, while providing a diagnostic modality that is versatile for a wide range of clinical and research applications.