9-(2-Phosphonylmethoxyethyl)adenine (PMEA), (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine (HPMPA), and (R,S)-9-(3-fluoro-2-phosphonylmethoxypropyl)adenine (FPMPA) are selective antiviral agents with activity against a broad spectrum of DNA viruses and retroviruses. A highly sensitive method has been developed to determine concentrations of these compounds in human plasma. Plasma samples containing PMEA, HPMPA, or FPMPA were treated with chloroacetaldehyde to yield the corresponding highly fluorescent 1,N6-ethenoadenine derivatives. Chromatographic separation combined with excitation at 254 nm and fluorescence detection at 425 nm allowed quantification of PMEA, HPMPA, and FPMPA within a concentration range of 0.25-4000 mumol/L. This method proved useful in accurately determining low PMEA concentrations in sera from PMEA-treated monkeys and cats. The assay should be applicable to the quantification of PMEA, HPMPA, and FPMPA in plasma and urine of humans treated with any of these drugs.