Up to the present time a specific assay method has not been available for detection of 2-aminoethylphosphonic acid (AEP), an unusual naturally occurring 13-amino acid with the unique covalent carbon phosphorus bond. A prerequisite for an investigation of the occurrence of AEP in human tissues is the availability of a sensitive and specific method for its determination. Such a method has been developed for the detection of AEP in biological materials.