Abstract: In the assaying of direct and indirect bilirubin, it is has been found that organic compounds, capable of undergoing diazotization to products having absorption spectrum substantially identically to bilirubin, can be used as a calibration standard. Typical of these organic compounds is ANS (8-anilino-1-naphthalene-sulfonic acid). However, these compounds do not have the typical yellow color of bilirubin. It was also discovered that this deficiency can be corrected by the addition of methyl or ethyl orange. When combined, these compounds provide a liquid control stable at room temperature for 12 months or more.