Abstract: Biological fluids were deproteinized and I was adjusted before centrifugal filtration (Amicon Centrifree filter; 20 min). The filtrate was analyzed for amino-acids by HPLC on a column (15 cm x 3 mm) of cation-exchange resin (7 µm; Li+ form) at 45°C with Li citrate buffers (pH 2.75 and 7.5) as gradient eluents (0.3 mL min-1), post-column derivatization with phthalaldehyde and fluorescence detection at 425 nm (excitation at 334 nm). Iohexol and trometamol were eluted near or on the tyrosine peak, phenylpropanolamine was co-eluted with tyramine in the following chromatogram and amikacin gave a post-arginine peak which carried over on to the subsequent 2 to 3 chromatograms.