In general, the present invention provides a reliable assay for the quantitative determination of p-aminophenol in an aqueous sample. More particularly, the present invention provides a rapid enzyme-based assay for the quantitative determination of acetaminophen in a sample. The assay employs a xylenol chromophore and a catalyst that is preferably a weak oxidizer. The assay provides reliable results in the presence or absence of N-acetylcystiene (NAC) and can therefore be used to monitor acetaminophen levels during NAC treatment. Methods and kits for determining acetaminophen concentration in an aqueous sample are also provided.