The method of Barker and Summerson (J. Biol. Chem. 138, 535, 1941) was modified to achieve reproducibility. Pyruvic acid is removed by 3 successive treatments with Cu_lime. The conversion of lactic acid to acetaldehyde is conducted by heating 1 ml. for 30 min. at 60[degree] in glass-stoppered tubes with 0.05 ml. 12% CuSO4 5H2O and 6 ml. H2SO4, incubating for 20 min. with the p-hydroxydiphenyl reagent, and destroying the latter by heating at 100[degree] for 90 sees. The colored complex obeys the Lambert-Beer law at 560 m/i. for 0-8 pig. of lactic acid.