tion of urea in blood serum byconcentrations of a formaldehyde source in the range of from about 0.5 to about 5 weight percent of the assay reagents and preferably in the range from about 1 to about 3 weight percent.The .beta.-diketone used in reaction (2) may be either a compound of the structure R--CO--CH.sub.2 --CO--R.sup.1 wherein R is an aryl group having from about 6 to about 20 carbon atoms