Abstract: An improved reagent combination of the type comprising a first solution comprising an aryl amine and an acid; and a second solution comprising a nitrite compound. The regent combination is characterized in that is further comprises a salt, wherein the salt is present in any preselected distribution in the solutions.Also, an improved method for colorimetrically determining bilirubin in a sample comprising mixing an aliquot of the above first solution with an aliquot of the above second solution to form a stable diazo reagent; contacting a sample to be assayed with a total bilirubin accelerator buffer system to form a prereaction medium; contacting the prereaction medium with the stable diazo reagent; and measuring the color of the azobilirubin formed form the coupling reaction of the stable diazo reagent with the bilirubin present in said sample.In addition, an improved method for producing a diazo reagent of the type comprising reacting an aryl amine with a nitrite compound in the presence of an acid.