An evaluation of a new colorimetric assay (SIGMA-430) for the rapid determination of acetaminophen in serum or plasma was performed. Acetaminophen was reacted with nitrous acid to form a 2-nitro-5-acetamidophenol derivative, which was measured colorimetrically in an alkaline medium. The assay was linear over a concentration range of 50-500 mg/L. Within-run and between-run CV's with a 300 mg/L sample were 1.51% (n = 12) and 3.00% (n = 20), respectively. Mean analytical recovery of acetaminophen added to plasma at 50-600 mg/L was 102.0%. Seventeen commonly prescribed weakly acidic or neutral drugs were also analyzed by the method with no significant interferences. A comparison of 21 consecutive serum samples with the colorimetric assay and an HPLC method yielded a correlation coefficient of 0.989. Colorimetric values were statistically higher.