This communication describes and evaluates an improved routine methodology for quantitating clinical proteinuria. Based on investigations of Piscator and of Savory et al., a modified Tsuchiya's reagent (ethanolic HCI-phosphotungstic acid) is used to precipitate proteins at 56 degrees C, followed by biuret spectrophotometry at 540 nm. The accuracy of the proposed procedure was assessed by comparisons with results obtained by using an ultrafiltration membrane that retains solutes with an average molecular weight in excess of 10 000 for separating of urinary proteins before they are measured with the biuret reaction. Precision of the method (coefficient of variation) is typically 2-3%.