Total urine protein was measured in 132 samples by an automated benzethonium chloride method and the Ponceau-S/trichloroacetic acid (PS/TCA) method. Of these, 27% gave a result 0.1 g/L or more higher by the benzethonium chloride method. Of this 27%, most contained an abnormally high concentration of the acute-phase reactant, α1-acid glycoprotein. By assaying urine containing added α1-acid glycoprotein and albumin, we found that α1-acid glycoprotein causes the PS/TCA method to underestimate the total urine protein concentration, whereas the benzethonium chloride method is unaffected. Not all urinary albumin was precipitated by TCA when α1-acid glycoprotein was present. Therefore, protein methods in which trichloroacetic acid is used as a concentrating step before the assay will underestimate total urine protein when the concentration of α1-acid glycoprotein is high.