The measurement of amino acids by high-performance liquid chromatography has become more common in recent years. Separation can be by gradient elution of the native amino acids followed by reaction with ninhydrin or by pre-column derivatisation with ultraviolet or fluorescent detection. The use of electrochemical detection for the measurement of amino acid derivatives formed with either o-phthalaldehyde or phenylisothiocyanate is discussed. A method for the assay of amino acids in human blood serum and urine samples using phenylisothiocyanate as the derivatizing agent and electrochemical detection is described and compared to previously reported methods based on o-phthalaldehyde derivatisation.