The extinction coefficient of .0026% uric acid in H2O or phosphate buffer solution (pH 7.4) has 2 maxima 1.46 at 2920 A., and 1.2 at 2350 A. In 0.1% Na2CO3 or Li2CO3 the 2nd maximum disappears, and the 1st shifts to 1.54 at 2950 A. In the ultrafil-trate of 13 normal human sera both maxima appear. The uric acid concentrations calculated from the extinction coefficient at the 1st maximum agree with average normal figures obtained chemically. In the serum of a patient with syphilis and carcinoma the 1st maximum appeared only as an inflection; the 2nd was absent.