Citrulline is an amino acid very similar in structure to homocitrulline; however, the former is one methylene group shorter than the latter. Cell Biolabs’ Homocitrulline/Citrulline Assay Kit is a simple colorimetric assay that measures the amount of total homocitrulline and citrulline present in biological samples in a 96-well microtiter plate format. Each kit provides sufficient reagents to perform up to 100 assays, including blanks, homocitrulline or citrulline standards, and unknown samples. Sample homocitrulline/citrulline concentrations are determined by comparison with a known homocitrulline or citrulline standard. The kit has a detection sensitivity limit of 37.5 µM homocitrulline or citrulline. The provided reagents are sufficient for the evaluation of 100 assays including standards and unknown samples.