Blood serum is tested for the presence of urea nitrogen by reacting urea present in the serum with an aqueous reagent having a pH of between 0.4 and 1.5 and containing diacetylmonoxime, phenazone, arsenic pentoxide and sulfuric acid. Reaction is effected by heating the serum-reagent mixture to 85* to 100* C., cooling and measuring the optical absorbance of the cooled mixture at a wavelength between 450 and 485 nm.