CTRP5, a 25 kD secretory protein, is a member of the C1q and tumor necrosis factor superfamily whose structure resembles adiponectin. A RT-PCR study demonstrated CTRP5 expression in RPE, liver, lung, placenta, and brain and it has been proposed that a CTRP mutation (S163R) plays a critical role in affecting its higher order protein structure, potentially leading to a cause of abnormal adhesion between the RPE and Bruch membrane. Recent data indicates that CTRP is one of the genes highly induced by elimination of mitochondria and able to activate AMPK in a rat myotube cell line, L6. Stimulation of L6 with recombinant CTRP5, full length as well as globular domain, enhanced glucose uptake and fatty acid oxidation. These biochemical events did not seem to be mediated via AdipoR1 or AdipoR2, suggesting that a novel receptor(s) may exist for CTRP5 in this muscle cell line. Some CTRP members can physically interact with adiponectin, forming various multimeric structures. Therefore, measuring serum or plasma CTRP5 may provide important information on its involvement in novel metabolism. This assay is a competitive Enzyme Linked-Immunosorbent Assay (ELISA) for quantitative determination of human CTRP5 in biological fluids. A polyclonal antibody recognizing native human CTRP5 reacts with a series of predetermined recombinant human CTRP5 standard proteins or samples under competition in the human CTRP5-coated plate. Their relative reactivity is plotted with that of the standard proteins. This ELISA is specific for the measurement of natural and recombinant human CTRP5. It does not cross-react with mouse CTRP5, human CTRP2 (globular), human CTRP6, human CTRP7 (globular), human CTRP9 (globular), human CTRP10 (globular), mouse CTRP2 (globular), mouse CTRP9 (globular), human adiponectin, human adiponectin (globular), mouse adiponectin, mouse adiponectin (globular), rat adiponectin, rat adiponectin (globular), human RBP4, human Nampt, human vaspin, human GPX3, human ANGPTL3, human progranulin, human leptin. The assay range is 0.001 – 5 µg CTRP5/ml and a detection limit of 1 ng/ml (based on adding two standard deviations to the mean value of the (50) zero standards).