Abstract: Diazonium ions which are useful as reagents for the assay of bilirubin content in a sample, such as a body fluid sample, are provided. In one preferred embodiment, 2-methyl-3-nitroaniline diazonium ion is provided, having the structure: ##STR1## Also provided are reagent compositions including the diazonium ions. The reagent compositions may be in liquid or solid form, and further may include other components, such as buffers, carriers, and solubilizers. Salts including the diazonium ion and a counteranion also are provided. Also provided are methods of using the diazonium ions in assays to detect or quantitate bilirubin present in a sample such as a body fluid sample. Using the assays, the amount of direct, indirect, and total bilirubin in a sample may be accurately and reproducibly detected, and optionally correlated with the presence or absence of any of a variety of diseases or disorders of organs such as the liver, gall bladder or intestines.