A new TLC separation and quantitation method for L-tyrosine and L-Dopa. mixtures was developed. The minimum tyrosine and Dopa quantities which can be measured by this technique are 0.7 and 1.5 mu g respectively. The method may be used to measure the kinetic parameters of polyphenoloxidase as well as to trace the enzyme catalyzed conversion of L-tyrosine into L-Dopa. For a set of 20 measurements the maximum difference between any two measurements of: spot areas was found to be 3 mm(2) (4.5%) for L-Dopa (4 mu g) and 2mm(2) (5.6%) for L-tyrosine (1 mu g).