Retinol binding protein (RBP) 4 is the only specific transport protein for vitamin A in the circulation whose function is to deliver vitamin to target tissues. In obesity and type 2 diabetes, the expression of Glut4 is significantly impaired in adipocytes. Glucose transport via Glut4 is the rate-limiting step for glucose use by muscle and adipose tissue. Adipocyte-specific deletion of Gluts leads to notable elevation of mouse RBP4 causing systemic insulin resistance, and that reduction of RBP4 improves insulin resistance. This identified a novel role of RBP4 in regulating insulin action and RBP4 is recorded as an adipocyte-derived hormone. The RBP4 (human) ELISA Kit is to be used for the in vitro quantitative determination of human RBP4 in serum, urine and cell culture supernatant. This assay is a competitive ELISA which utilizies a 96-well microtiter plate which was pre-coated with a human RBP4. A purified polyclonal which recognizing native human RBP4 reacts with a series of predetermined recombinant human RBP4 standard proteins or the test samples under competition in the human RBP4-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 RBP4. It does not cross-react with mouse RBP4, rat RBP4, human adiponectin, rat adiponectin, human resistin, human vaspin, human clusterin, human leptin, human IL-23, human IL-33, human GPX3, human Nampt, human ANG1, human ANG2, human ANGPTL3, human ANGPTL4, human ANGPTL6, human FABP4, human RELM-beta, rat RELM-α, mouse Nampt, human PAI-1. The assay range is 0.001 – 5 µg RBP4/ml and a detection limit of 1 ng/ml (based on adding two standard deviations to the mean value of the (50) zero standards).