Methods for the fluorescence derivatization of cardiac glycosides with concentrated acids from TLC are adopted to HPLC for post-column derivatization. The column effluent is blended with concentrated acids in a knitted tube reactor, which enables derivatization with negligible increase in chromatographic peak width. The selectivity of the reaction is temperature-dependent and influenced by the respective acid. Reactivity increases from H3PO4-->CH3SO3H[symbol: see text]H2SO4. The conversion of digoxigenin, digitoxigenin and their digitoxosides is accelerated by Cu(II) acetate or Co(II) nitrate in H2SO4. Combined with a new two-mode, single-column solid-phase sample preparation, cardiac glycoside levels of less than 100 pg/glycoside in 1 ml plasma are detectable.