Abstract: Monosodium 4-amino-1-hydroxybutane-1,1-diphosphonic acid (MK 217) (in tablets or solution) was diluted or extracted with water and injected into an IONPAC NS1 5 µm column (Dionex) which was packed with a neutral macroporous resin that was stable over the entire pH range. The mobile phase (1 mL min-1) was 10 mM phosphate buffer (pH 8.0) - acetonitrile (3:2) containing 10 mM cetyltrimethylammonium bromide, with post-column derivatization (with o-phthalaldehyde and mercaptoethanol) and fluorimetric detection. The calibration graphs were rectilinear from 0.1 to 10 µg mL-1 and 10 µg mL-1 to 1 mg mL-1 with a detection limit of 100 ng. Within-day and between-day coefficient of variation at the 5 µg mL-1 level were 2% and 7%, respectively.