Dimethylaminoazobenzene-4'-Thiohydantoin (Dabth)-amino acids have an absorption maximum at 436 nm. This obviates interference from UV-absorbing impurities and baseline rise due to alterations of the solvent gradient during high performance liquid chromatography. All the DABTH-amino acids, except those of leucine and isoleucine, were satisfactorily separated by Thin layer chromatography on a two-dimensional polyamide sheet. The sensitive azo group allowed the detection of DABTH-amino acids as red spots down to picomole amounts. DABTH-Lysine and DABTH-arginine did not leave the column. The quantitative determination of the derivatives at picomole concentrations in the relatively short time of 30 to 40 min was possible. Foriers et al. demonstrated that an Altex Ultrasphere Octadecylsilane column operated at 32°C efficiently separated the common DABTH-amino acids. The success of the Edman method for the sequential degradation of peptides has led to the synthesis of modified isothiocyanate reagents designed to improve the ease and sensitivity of the procedure.